Now you could spend good money on a kit, but hey... I'm cheap. I didn't even spring for the pink foamboard. And be weary of adding too much extra weight.
Know your door. Who made it, and what type of springs it has.
Adding weight to your garage door may require you to adjust your springs. I recommend torsion springs be handled only by a professional. If you want to adjust an older extension spring style door, do your homework . If you are wondering the difference between the two... Call a professional!
I purposefully kept the weight down by not adding adhesive or caulk. This works simply because you are reducing the surface area of cold metal that radiates into the room.
Step 1: Materials
1.5" or bigger for a tight fit. Pink foam cuts easier and has a better R value per inch.
I buy the cheapest ones, as they just go dull fast using them like this anyway.
A new one works best if you use the white foam board as I did, and plan on it being disposable.
straight edge and tape measure.
foam board adhesive.
Step 2: Measure... Measure... CUT
Mine hold them selves in. And don't try to be perfect. The main principle here is to reduce the surface area of cold metal because that is what makes the biggest difference. Even the weather seals around your garage door are not perfect, so do what you can and you will still see a huge change.
I have cut the panels about 1/2" smaller before and glued in the panel with adhesive, then used spray foam around the edges. The biggest drawback's for that way are the mess, the ascetics, and the need to adjust the springs from extra weight.