I completed the Form 1 but not certain when I'm supposed to engrave the lower. What goes on there? Should I do it while waiting for the Form 1 approval or will they assign a number to be engraved? Where do I go for engraving? Lastly, this for an AR, where on the lower is best if I really don't want to look at all the time?

You can engrave anytime. There is no regulation forbidding possessing an engraved receiver with out a permission note. Just don't install a short barrel on it, or have a short barrel without a stamp on a receiver in your safe.

Assuming you paid the tax, engrave any time before "making" the SBR. Companies that regularly do NFA engraving can usually meet the legal requirements by engraving in an inconspicuous place like trigger area, bottom of the magazine well, etc. The rules say it has to be visible, so hidden under the pistol grip or inside the receiver is no good, but the rules don't say they have to be obvious.      

If you applied under Final Rule 2021R-08F you can adopt the manufacturer's marks and skip engraving. This can be a big advantage if the gun has collector value that would be reduced by non-factory engraving.

The engraving consists of the name of the maker (individual or trust, as applicable) and address (city/state).  It should match the info on the Form 1 you submitted.  For example “John Smith Miami, FL” or “ABC Trust Miami, FL”.

I'm about to do my first form 1. Already have an approved form 4 for a suppressor.

I will register as an individual and engrave accordingly. What if, in the future, I want to put both into a trust? Will I have to re-engrave the receiver?

The maker is required to engrave. Subsequent transferees are not.

I don't think you're in violation of anything if the maker didn't engrave, and now it's registered to you, but nobody ever talks about that scenario. Reading the statute, it's just like 922(r). Nothing specified about possessing something un-engraved or assembled without regard to US parts count. Those regs only apply to whoever does the making.

THSF can engrave in several different locations. They’re awesome to deal with. Super quick turnarounds usually too.

Local may be an option for you, just verify they do good work. I can understand wanting to avoid the hassle of shipping and going local, it just wasn’t an option for me.

I made the mistake of trying to find someone local on one lower.  Since then I’ve been a fan of THSF

Thank you for clarifying.

FYI, you can't just "put" it into a trust later - that's another transfer and stamp. If you know it'll end up in a trust, best to just start out with a trust.

ETA: @KitBuilder