I sat my safe on top of a piece of plywood, on top of the carpet. I didn't want the bottom of the safe to possibly be dirty/rusty, and stain the carpet. I cut the baseboard where the safe was, and put the safe flush against the wall. I measured, and ran lags through the back of the safe into the wall studs.
On the bottom, I drilled holes through the bottom of the safe, and through the carpet / floor. I ran long bolts (all thread) down through them. In the crawlspace below, I used sections of this metal framing strut/channel stuff:
to span across a few joists. I then ran the bolts through this, and cinched it all up. I never drilled into the actual floor joists (didn't want to weaken them), but in essence I "clamped" the safe to the floor joists/subfloor/carpet/plywood below.
The safe lived there for a couple of years. When I pulled the safe (when we moved) the carpet under it fluffed right back up, and looked completely normal.