First, you can't legally even buy a Full-auto type lower receiver.
All available lowers are made as semi-auto ONLY, and won't accept the necessary full-auto parts.
All you have to do is NOT deliberately attempt to convert the rifle to full-auto.
To do this, when you assemble your lower, use ONLY semi-auto parts.
This would be a semi-auto trigger, disconnecter, safety, and hammer.
The bolt carrier in the upper can be a semi-auto OR full-auto type part, and this is perfectly legal.
There's at least one good large size paper back book out that covers how to assemble an AR-15 rifle.