An AR-15 pistol must be built on a lower receiver that has never been used as a rifle. You cannot legally just modify a rifle to become a pistol!
There are lots of different options for a pistol build. A non NFA pistol is not legally allowed to have a stock. There are several pistol braces available that look similar to a stock, but are legally not a stock. Most of them just attach to the pistol buffer tube. I like the KAK Blade because it's light and simple. I've built a few AR pistols, and other than my 300 Blackout, they are cool looking but a bit disappointing performance wise. The .223 round loses a lot in a pistol length barrel.
If you want to build one, remember you need a lower that is either brand new or from another pistol to stay legal. I usually buy my lowers from Juggernaut arms. I've also bought a few 80% lowers and completed them myself. My lightest AR pistol is built on a polymer 80% lower and a carbon fiber hand guard. It is very fun to shoot.