If you go the lawyer route, which I did...
The best method is to order finger print cards from
BATFE forms web site (link embedded). Fill in personal information, then click on Fingerprint cards. You need card "FBI FD-258LE" for NFA. You need TWO finger print cards per NFA device application.
Generally you can go to your local police department or county sheriff's office to be fingerprinted. Some charge a fee, some do not...my sheriff does not charge to be printed.
There are plenty of apps (free) for iPhone and Android devices that take an image in proper passport format. Barring that, most Walgreens or UPS Store can take a passport photo for you.
I recommend delivering a notarized copy of your trust, fingerprint cards and passport photos to your local FFL/SOT who will be doing the transfer for you. Having one person do all the paperwork makes mistakes less likely.
Some online vendors will go ahead and submit the Form 4 on your behalf. Once submitted, NFA Branch issues a Control Number. You can then personally mail the trust, fingerprint cards and passport photos to NFA Branch referencing the Control Number. But again, I am aware of issues going this route; rare, but they still occur.
If you are purchasing from the SilencerShop, and using a lawyer created trust, you will likely need to FedEx a copy of the trust and fingerprint cards to them. You can probably email the passport photo once scanned.
Some FFL/SOTs now use the digital fingerprint process which makes ink/paper finger print cards unnecessary. This is super convenient as nothing needs to be mailed to NFA Branch. The review process begins immediately once all docs are submitted electronically.
I used to be a Type 07 FFL and Class 3 SOT holder. eForms have greatly helped submitting to NFA Branch. But eFroms and NFA Branch are still going through growing pains. When an individual's forms/docs come in from different sources, mistakes happen. I typically recommend using a local FFL/SOT to do everything. Especially if your FFL/SOT has invested in the $1500 digital finger print scanner approved by the FBI. All required forms/documents get electronically submitted at one time. If you end up mailing docs in to NFA Branch, and they arrive on the 15th day after the eForm 4 was submitted, the entire package gets revoked, requiring resubmission of everything. A RPITA!