To clarify, military tactics didnt just change from Napoleonic and Franco-prussian wars to modern or Second World War tactics so obviously the troops of the time had to experiment. What were THOSE tactics like?
For example, in the final battle of the Isonzo River, German soldiers were applying the changing tactics along with the Austrians to defeat the Italians that were heavily entrenched in the mountains. What did they do? Were they applying small unit tactics that are used today?
I will most likely make many mistakes in the naming of the tactics, I learned them in French and my translation will not correspond perfectly to the ones used in English. I'll try to be as just as I can, but don't take my naming for the "official" one.
Many tactics were created during WW1, and many uses of the new firearm/cannons were innovated upon.
For example at the begining of the war, cannon fire was used just before an assault, and stopped to let infantry attack. During the war the "Rolling bombardment" was developped, the idea was to use cannon fire as a barrage to protect your own infantry. Slowly advancing where the cannon shot in order to always have your opponent underfire while your troops were advancing into the opponent trench while they were "stunned" from the explosive barrage they just took.
It failed on many occasions due to poor planning, and problem to send the information on where your infantry was, so the front of the bombardment rapidly outpaced the infantry, leaving the defendant time to get their bearing back and prepared for the assault.
Trench warfare saw huge "improvement" during the war, from a system of a few trenchs not really thought upon to the German system at the end of the war. Which was based upon the idea to not let most of his troops in range of bombardment, so they put as few men as they could in the front line, then spread many machinegunner behind this frontline, these men would be in reinforces holes, somewhat comparable to proto-bunkers. Most of their troops would wait for the British/French to advance deeply into their lines, then counter attack when they were tired. It proved highly succesfull.
Germany also developed the "Deep penetration" strategy, using a few men going in as far as they could into enemy line in order to disrupt the defence as much as they could. These unit weren't supposed to be scared about being encircled because the rest of the infantry should deal a decisive blow to the rest of the frontline and follow quickly. It was highly succesfull against Russia, but didn't worked so well against the better organised French/British, it still prove somewhat succesfull.
Finaly we have the uses of Tanks slowly developping during the war, it was pratically only used by the Allied. Their first use of tank was to just tell them to go with the infantry in a standard attack. It proves quite succesfull. But with the war progressing, the allied started to have more and more of them (a few hundreds by 1918 if I remember correctly). And started to incorporate them into better thought strategy.
The allied used it after a heavy bombardment against Germans position, and put many troops behind the armored vehicules to protect them. These attack used at the end of the war were the less devastating on the attacker side of the war.
WW1 saw incredible change in military tactics, it's really the "inbetween" war in term of military tactics, having older tactics not working anymore because of the increase in firepower. But new ones hard to create effectively because of the lack of mechanised units du to technological limits of the time.
John Keegan "World War 1" might interest you on this subject, he goes through WW1 looking at how the war was fought, and focused highly on the military perspective. It seems to be a great way to introduces yourself into the subject. Moreover he list dozens (maybe more) sources, leaving you with a great deal of new books/research to look into if need be.