To address something earlier... always assemble our lower and test it first and foremost. Make sure it fits & functions 100% flawlessly because you will likely be married to it for the rest of your life so SBRing a lemon would be a real bummer. Up next... engrave. If you send it off or have it done locally always engrave first. I use a place in MN called Orion that charges I think $35 for front of magwell (filled) and return shipping of like $14 or something... cheapest I found and that was comparing to others that use a standard side-of-receiver location that still cost more. Front of magwell is way more tacticool. lol Then, once you have your lower engraved fill out your paperwork FROM THE LOWER. In other words, don't send off your form first because if you get approved and then have your lower engraved you not only have more rules to adhere to now since it's technically a NFA item but you also MUST get it exactly as the form read when it was approved. I had one situation where the engraver added a period after my first initial even though I explicitly requested it not have any punctuation. I got the lower back and noticed the addition and wasn't upset at all since I hadn't yet filled out my paperwork. Completed the paperwork from the lower receiver itself (with the error) and sent it off and never gave it another thought. Now, had I already been approved w/o the period... different story. Your engraving technically doesn't match what you stated it was on your form. When you fill out your form do some Googling and read discussions regarding this topic. Take what everyone says with a grain of salt, move on and read the next one. Compile all the discussions and make your own decision based on those how to fill out your form. The picture doesn't have to be an official passport photo as those are expensive. Stand in front of a white background in good light and have someone use your smartphone to take the shot. Use an auto-edit program to make it look professional & then trim it to the proper crop as outlined by passport photo guidelines. I've done several like this and even send the same pic every time and they take it every time even though the picture was taken several years ago. Finger print cards are the same thing. Get several done as your local LE will likely charge you one free for 2 or 10. If you don't have the BATFE approved finger print cards use the same one that your local LE uses but don't fill out the BATFE's info. They prefer you send it blank so they can use their stamp for their own address & info. Also check with your local LE where you plan to have your fingerprint cards done because most these days have a fancy printer that will do all the work for you. They ask you all the info on the card, input it into the computer, electronically capture your prints and then print out as many cards as you need with all of the pertinent info auto-populated. So, call ahead before you waste an hour hand filling out all of your own info on print cards.
I'm sure there's more but that's about all the pro tips I can think of right now.