I'm sure everyone has their way of doing things but this is how I built my first one.

1. Decide what you want to build and acquire the lower or pistol.
2. Get paperwork filed either efile with a trust or paper individually.
2a. If snail mailed what about two months and call ATF to see if it is pending.
3. If you have a pistol go play with it for a while since the paperwork will take anywhere from 4-11 months.
4. If you have a lower you can build it up to the point of needing the upper. You can't legally possess a barrel until you have the paperwork back from the ATF.
5. Wait
6. Wait some more
7. You can get the lowered engraved at anytime some will wait till the paperwork is back others do it before hand. Good and bad points either way.
8. Paperwork arrives make copies so you have the original in the safe and copies with you just incase someone asks for it.
9. Go play with new toy.
10. File paperwork for another because you just can't have one NFA toy.