For the two battles I've mentioned, I read their Wikipedia articles, and the ways the preparations are described seemed pretty basic:
intelligence received of the enemy fleet --> ships are prepared --> fleet departs to engage the enemy
Surely there is much more to it but the Wiki articles don't go in depth about the preparations (yes, I'm aware Wikipedia is not the most reliable source but it's the only thing I can access now):
Less than a week after the Capture of Gibraltar, Admiral George Rooke received intelligence that a French fleet under the command of Toulouse and d'Estrées was approaching Gibraltar. Leaving half his marines to defend the newly won prize, Rooke immediately set off with his combined Anglo-Dutch fleet to engage the French.
The same month, Nelson returned home to Britain after two years of duty at sea. He remained ashore for 25 days and was warmly received by his countrymen. Word reached Britain on 2 September about the combined French and Spanish fleet in Cadiz harbour. Nelson had to wait until 15 September before his ship, HMS Victory, was ready to sail.
On 15 August, Cornwallis decided to detach 20 ships of the line from the fleet guarding the English Channel and to have them sail southward to engage the enemy forces in Spain. This left the Channel drastically reduced of large vessels, with only 11 ships of the line present. This detached force formed the nucleus of the British fleet that would fight at Trafalgar. This fleet, under the command of Vice-Admiral Calder, reached Cádiz on 15 September. Nelson joined the fleet on 28 September to take command.
I have several follow-up questions further regarding intelligence, preparations, and fleet departure (which are focused on the Age of Sail, but because it is a wide time period, I will narrow it down to the 18th century):
Intelligence:
when Rooke received intelligence and when word reached Britain after Nelson returned there, how were these two men actually told of the news? Who, specifically, would have told them? (naval agents? spies? some other colleagues?) Would they have been given letters, or was the news told verbally because letters might have risked being intercepted by the enemy?
Who were usually the first people to know of an enemy fleet and spread the intelligence?
Was there some sort of organized way that the news of an enemy fleet to be fought was spread? Down from the head of the navy to the admirals, to the ship captains and to the lowly sailors?
preparations:
what was the first action that was done after intelligence was received? Were there decisions on which ships should join a line of battle fleet? (e.g. "HMS Portsmouth under Captain Smith should join the fleet") What responsibilities and duties did the admiral(s) or other high-ranking naval personnel have?
Before a fleet was to even depart, were there plans drawn up (like, on paper?) and discussed for how the fleet would physically approach the enemy? If so, which people in the navy would be involved?
fleet departure:
Wikipedia says that Admiral Rooke immediately set off with his combined Anglo-Dutch fleet, but just how long would it have actually taken to depart with a fleet of 53 ships of the line? Did all the ships depart at once, or did smaller sections of the fleet departed one by one and wait for the others to reach the sea? I've read from another thread on this sub that 18th-century fleet preparations took a very, very long time.
Wikipedia says that Cornwallis decided to detach 20 ships of the line from the fleet. Though being an admiral, was this his own decision he was able to make fully on his own, or would he have first had to seek permission from someone of a higher position in the navy? (I honestly do not know if there are any higher positions than 'Admiral' in the Royal Navy forgive me for my quick assumptions)
I know my question is loaded but this is a topic that I have been interested in for a while and would love to read any answers received
(First-time commenter, please excuse any errors in formatting)
The generous amount of resources available to us regarding the leadup to the Battle of Trafalgar allows us to paint a fairly good picture of what naval intelligence looked like in the Age of Sail (although it should be noted that a majority of these resources are concerned with the British side of the engagement, so these systems could look vastly different for other nations). The process of intelligence gathering was done by a string of frigates spread out to the horizon, allowing a set of five frigates to warn of French movement along a 200-mile observation chain. Paired with a comprehensive signal book that had been updated not more than five years prior to Trafalgar, British vessels could set up massive nets to observe the movements of vessels along the French coast with relative ease. This system was used by Horatio Nelson to pin the French fleet under the command of Pierre-Charles Villeneuve in Toulon between 1803 and 1805, using the frigates to spot whenever the fleet would attempt to leave port while keeping his primary force stationed deeper in the Mediterranean to move and intercept any escape efforts (and also out of sight of the French so that they couldn't know when the British fleet had been blown off station, something that happened quite often).
This blockade was finally broken on March 30th, 1805 when Nelson, in an attempt to bait out the French, allows his fleet to be spotted off the coast of Spain. This information reaches Villeneuve who breaks out of Toulon and sails for Sardinia in an attempt to circumvent the British fleet, while simultaneously the British fleet moves to Sardinia to cut off the French and force them into a decisive engagement. Villeneuve's force comes across a Spanish merchant who tells them that the British are lying in wait so he turns west and makes a run for Gibralter with Nelson, now out of contact with his frigate net, only finding out that the French had cleared Gibralter on the 16th of April. Nelson's intelligence gathering efforts begin to deteriorate as he sends himself in pursuit of the French fleet which has made sail for the West Indies, forcing him to rely on merchant vessels and colonial garrisons to catch glimpses of Villeneuve's force.
This goes on for a month, the British fleet hopping from island to island trying to find news of the French, only to be given bad information until finally, he receives word that the French fleet is heading back to Europe. In hopes of catching the French before they can return to port, he sends the HMS Curieux ahead of the fleet in hopes that it could reach the Admiralty before Villeneuve can find shelter, which it did on July 9th. The Admiralty then sends word to Admiral William Cornwallis to raise the blockade at Brest and send half of his forces to patrol Cape Finisterre under Admiral Robert Calder and another half to patrol the Bay of Biscay, hoping that one of the forces will catch the French before they can meet with any of their channel squadrons.
________________________
I just want to give a brief interjection regarding the relationship between the Admiralty and the actual admirals serving under them. The sheer size of the British Navy, which at the time operated around 136 ships of the line and another 160 cruisers across four different naval command groups, those being the North Sea, Channel, Mediterranian, and Baltic, required some central point of command and communication, which was the role of the Admiralty. More specifically, this job fell on the Lord High Admiral who was considered responsible for the strategic actions of the navy as the head of the Admiralty. However, due to the nature of nautical scale, these orders were often quite broad and more specific decisions and actions were left to the admirals and their good judgment. There was also a major issue with timing since unless you were like Nelson (or Cornwallis, who had operated a similar frigate net in the Channel), there was a very good case that your orders could be received quite late, causing items like tight timetables to fall through. Finally, I should note that obviously, the higher in position you are, the more freedom you have in deciding how to interpret orders given by the Admiralty which is what allowed Nelson, as the commander of the Mediterranian Fleet, to completely break from his post and pursue Villeneuve's fleet to the West Indies and back.
________________________
Admiral Robert Calder engages the French at Cape Finisterre (an action so lackluster and indecisive that it is referred to simply as "Calder's Action of July 22nd") and Villeneuve breaks for Coruna for refitting after nearly three months at sea. After a short period, the fleet once again sets off for Breast but turns back after Villeneuve mistakenly identifies a set of British frigates as scouts for the Channel Fleet, causing him to turn tail to Cadiz for proper repairs, arriving on the 20th of August, pursued by a small squadron led by Admiral Collingwood and later to be reinforced by Admiral Calder and the ships of the Channel Fleet dispatched by Cornwallis. This would be the situation until the 28th of September when Calder would be relieved by Nelson, who also had commands from the Admiralty to send Calder back to England for a court-martial in regards to his failure at Cape Finisterre.
From here, there isn't much to talk about in regards to intelligence. Captain Henry Blackwood would keep a watch over the French-Spanish forces in Cadiz, using a chain of frigates to report back to Nelson, who was keeping his main force over the horizon so that their location could not be reported back to the French (indeed even at the day of Battle, Villeneuve is rather unaware of the location of the primary British force). In this position, Nelson would also give orders to his captains regarding the order of battle that he wished to pursue once the French fleet left port, bringing his officers onboard his flagship Victory to discuss his plans, later also giving written orders which were rowed between the vessels as they sat in wait. These written orders are quite interesting as he lays out the double column strategy along with initial actions that lines should take, but leaves most of the actions past that up to the discretion of the individual ship captains.
Of course, we know the rest of the story, the French-Spanish forces leave Cadiz on the 20th of October and are engaged and defeated the next day but not without significant loses, along with the death of Horatio Nelson. I apologize if this answer is more about the story of Trafalgar rather than directed at the actual question, but I think the context helps answer a lot of what you are asking. Despite having advanced signaling guides, most of the intelligence gathering done was still through word of mouth, letters, and the sheer luck of ships arriving first. When communication systems worked, they worked, but more often than not, they just broke down. The sea is absolutely massive and attempting to get any sort of information across it is a herculean feat with the technology available, and it is amazing that everything worked out the way it did. I hope this answer has given you some good information about how the lead up to these types of engagements and the amount of work that goes into getting to the line of battle.