While playing the Total War series it always struck me as odd about how my units of infantry would always automatically conformed to my order instantly no matter how far away they were from the general's unit. During this time how did the army commander relay orders to units of infantry on the front lines?
Orders are always given from on top down. While I am too lazy to find the post I made about how orders are given, I shall tell you in full, however I would request that you look at the response for /u/Rosstafarii as I will omit the battalion level orders.
All orders in their gauge sense came from Napoleon himself. He would do one of two things, ether dictate orders or scribble orders on a paper to his Chief of Staff, Marshal Louis-Alexandre Berthier. Berthier would translate his orders (both literally and figuratively) into viable commands that would be given to every corps commander.
A moment on Berthier. Berthier is the unsung hero of the Napoleonic era. Napoleon is a genius but without Berthier, Napoleon wouldn't have the men where he needed them. According to John Elting in Swords Around a Throne, Berthier knew almost down to the exact man how many men were where and if they were marching, when they would be there based on his memory. Clearly Berthier was a genius in his own right and able to process inordinate amount of information that Marshal Soult (Napoleon's Chief of Staff in the Waterloo campaign) couldn't.
Berthier would then have orders drafted for each Corps, which would either rely on messengers from the aides of Berthier or from aides left at the Headquarters. From here it would be delivered by horse to the Corps commander, whom would read the order and disseminate the orders on down. This process would continue right down to the battalion level as battalions were the smallest tactical unit in the army at the time.
Edit for sources.
Swords Around a Throne by John Elting
The Campaigns of Napoleon by David G. Chandler
Battalion commanders would have some limited autonomy in fire control and presumably formation such as perhaps minor wheeling or forming square, but mounted runners would carry written notes from the top down. This was practiced even to WW1 and probably in some lower-level scenarios in WW2, when smaller units had more independence and radio communications may be easily disrupted.