Found this in a facebook group and thought I'd share it here.

http://www.instructables.com/id/DIY-MREsa-tutorial/

Edit to add:

If you don't have access to a vacuum sealer, simply zip the bag shut leaving about an inch unsealed. Submerge that bag in water until only the unzipped corner is exposed. Then finish zipping the bag. The water pressure will force the air out of the bag.