Start by making a ATF user account here
https://eforms.atf.gov/register/update-profile?isRegistration=trueyou need to decide if you want to make an NFA trust, this is an excellent utility, or file individual, John Pierce does $100 Trust
his site has excellent resources
https://johnpierceesq.com/National gun trust has good self help resources for filing, I'd suggest using there EFT e-fingerprint service
https://www.nationalguntrusts.com/products/eft-fingerprint-file-servicethen you can up load your prints into your application, without having to get 2 sets of prints and mailing them to Martinsburg within 10 business days every time.
you are not allowed to make corrections once you file it runs full course you get disapproved and start over at square 1.
do not submit cartridge as caliber, 9mm is 9mm, not 9x19, you will not impress them, they will disapprove you, 9 unit of measure is mm, .300 Blackout would be .30 unit of measure inch, the barrel or receiver must be marked the exact caliber as your application .223 is .223, 5.56 is 5.56
Overall length is maximum not minimum, from butt to end of permanent barrel
if you don't know your barrel length, remove muzzle device rod the barrel to the bolt face.
this is the ATF barrel rod test for barrel length.
for the mugshot I use a selfie with a white door as a backdrop, nothing fancy.