I installed one about 5 years ago. Bought a kit, either at Home Depot or
Menards, can't remember. For wall space, you need to have 2x the door entry
space, plus some constant, like 6" (don't quote me on that constant). In
that space you can have no plumbing, electrical, HVAC, etc -- it has to be
clear. You actually frame out the rough opening as if it was the 2x+ size,
then put the kit in. At least that's how my kit worked. The kit included
some metal "half studs" that you hung the sheetrock on over the "hidden"
part of the door.

With my kit, you bought the kit, plus a standard door blank and trim -- and
that was everything you needed (except for the handle hardware).

-Tim