OK, so, the best way to do this is to think about the main sub-assemblies of the firearm, and then go from there.
So, for an AR-pattern rifle, you have 2 basic sub assemblies:
The Upper Receiver and the Lower Receiver.
The upper receiver is a non-serialized part, so every component can be purchased on the internet.
There are 3 essential components of the upper: the Barrel Assembly, the Upper Receiver Assembly, and the Bolt Carrier Group assembly.
From there, a mat like this is your best friend: