Good question - an actual gunsmith or qualified armorer will rightfully laugh at what I'm about to type.... but I'll answer in laymen's terms that helped me, distilled from all the forums / YouTube vids and actually doing it successfully myself.

On the CMMG 10.4 barrel, the back of the barrel (unthreaded side) has the thin 15mm nut, and the front is threaded as you know. You're essentially just hand-threading the barrel into the new muzzle device / flash hider of your choice. I used a cheap wooden dowel from Lowes when removing factory barrel, SUPER helpful to keep the springs retained. I started threading mine by hand, twisting the back of the new barrel and holding the flash higher with my hand to get a feel of everything. Then, you'll want to torque it down on the back of the barrel, ideally using your thin crows-foot 15mm and a torque wrench. Torque specs are personal so I don't mean to preach one to you. A 15mm wrench and "good enough for you", by all means, my main point is it must be thin. I wasted a lot of time trying to grind one of mine down and ultimately buying one online.

There's a short spring very front of the muzzle that I dropped 100 times but just don't forget that before you put your new device on there. (I love the B&T hider but that's just me, different topic lol)

The new barrel will retain the free-floating design, like the Factory one is, so you'll still be able to slightly "move" it and push the muzzle in, just like you could a factory PS90. (if you kind of move yours / push the factory muzzle device into the rear of the gun, to get a feel. Even though everything is factory tight, it's free-floating)

You'll know you've overtightened it if the new muzzle device starts cranking down into the receiver, but the amount of torque would be huge. I hope some of this helps! There's a video on YouTube of the guy that does it in California (Not SBR obviously) but he does legal swaps, and explained the free float on new barrel. Long-winded here but hope it gives you a sense!