I used Utube vids and already having an AR to help me along when I first started years ago. One was to get the idea on how to do it and proper order to do it in, and the other to see how the finished item looks like, then testing to be sure each part does what it was suppose to do, like if it has a spring to make work, that the part springs back/forward as it should.
Your biggest head scratcher at first will be in how the springs wrap around and hold themselves on the hammer/trigger holding pins. When testing after done, make sure you don't let the hammer slam into the pocket wall in front of hammer. It could crack if the upper assembly isn't attached.
Just put your finger(piece of thick leather/rubber/carboard/flat stick) somewhere with support in front of hammer to not let it hit the pocket wall when testing, so after pulling the trigger to know that hammer only hits finger or other item with force, to know it'll work when using live rounds. This is also where you test the safe/fire switch, to be sure trigger either releases or locks trigger in place to not fire.
If everything on lower assembly works, just relax the hammer to make sure it isn't cocked back in firing position and it accidently snaps against that wall at any time while the upper assembly is off.