I don't fly indoors as I think its stupid and the chances of going pear shape is high. Here is a few pointers if you do decide...
Do NOT calibrate the compass indoors.
In the Phantom assist, turn off RTH = 0 meters and set to hover and land. Why?? Battery and failsafe triggers. Sometimes the phantom can acquire GPS signal through the walls and can do RTH on low battery. Last thing you want is the phantom to shoot into the ceiling on a false attempt to RTH.
As mentioned, fly in ATTI. Practice ATTI mode outside so you know how much control you really have. You will discover how much the GPS is a godsend and inertia is NOT your friend. Also, try and assure zero or minimal GPS counts by placing some foil on the outer centre top shell. This blocks the GPS antenna and enforces ATTI mode only.
Keep an eye on open windows, vents and draughts.
And finally keep away from walls and ceilings. Walls creates vortexes and causes the phantom to tilt. Ceiling creates suction and causes the phantom to suck up into the roof. Don't panic and the props are NOT angled like the P3 motors. Just descend slowly and DONT trigger CSC like many have.