Just did mine:

Check your local code so you don’t get screwed and you have resale value. Added egress window since one room was bedroom 2” XPS glued to walls 1” XPS on slab with double layer overlapping 7/16 CDX plywood. XPS will act as thermal and vapor barrier Concrete screws on first layer of floor ply and wood for second. Floor allowed 2x4 construction without wood on concrete. Use a laser square since basement walls are not square. Snap an offset chaulk line and find your min gap. That’s your 2x4 wall offset Closed off mechanical room since AC and boiler make a lot of noise 48” opening to mechanical room with barn sliders for access Rockwool insulation Fire block the wall top plate I added a heat loop off boiler but in the end I didn’t need it. Room stays 72 in winter

Do it right for the long run