" Place unopened pouch in warm water for 5-10 minutes.
Unopened pouch may be laid on a warm surface.
Lay unopened pouch in direct sunlight. Not much chance of that down here.
Place unopened pouch inside you shirt, allow you body temperature to warm your MRE.

I was surprised they left out: Place unopened pouch on ground and pee on it. "

― S.A. Bodeen , The Compound (The Compound, #1)