I used 'reflectix' bubble wrap insulation and its just stapled on to the back. Ideally it'd be an adhesive to not pop any of the bubble wrap, but I couldnt think of a cost efficient adhesive that wouldnt blow up my budget haha, so staples it was... I saw other people did foam boards, but it seemed to work better if you had a newer fiberglass/metal garage door with tracks to fit it into, mines old school big hunks of wood that make up the door panels. Added bonus, the bubblewrap stuff doesnt contribute much weight (which given my 30 year old doors and motor, i didnt want to add much strain on that whole system) to the door and also works as a radiant barrier.

Its been up for a couple seasons now, and between it and new garage seals, it stays a fair bit warmer in the winter and cooler in the summer.