This comes up pretty often, thought I'd post some pictures.
The 1" thick foil faced yellow foam board sold at Home Depot for 9$ a 4x8 foot sheet or so, is vastly better than styrofoam for this. HD actually sells kits of pre cut styrofoam panels for this, but they didn't actually measure the same as my door openings, and would have to be cut again anyways... at 5 times the cost.
Also, the R value for the foil foam is much better than styrofoam. And lastly, it is much easier to work with --cuts in 2 passes with a utility knife, leaves no mess... not like all those little styrofoam balls. And no static making all the bits cling to you like styro.
Measure the panel openings, then measure the full height of the inside, which will be like 3/8" more due to lips on the panels. Mark your foam, cut your first panel full size and see if you can fit it in by sliding it UP into the slot, then pushing it in. Probably not. So with the foam pushed fully up, mark the bottom edge of the foam so it will pass over the top of the lip, then trim that edge (probably 1/4" or so). Fit it in.
All my panels fit in tightly, but for some reason the bottom panel is different--taller lips. So on THAT one, I just cut the foam to the full height, then free-hand cut a slice horizontally across the foam part way through, and then just folded the foam board in half along that cut. So it is shaped like a V. Then just push the two legs of the V folded foam panel into the top/bottom garage panel lips, and push into place. The foam snaps in there with no desire to unfold, and the cut virtually dissappears.
You can do a single garage door with 2 4x8 foam panels, about 16$, and it takes 15 min or so.
If you want, you can open/close the door a few times so the foam panels are resting fully OUT against the lips (not pressed in up against the garage door, leaving an air gap). This gives better insulating properties, and is the way they will naturally rest. Then just run a can of "Great Stuff" expanding polyurethane foam around the edges of the individual panels to "seal" them in place with no air leakage, clean up the expanded foam later with a knife.
Works great, dropped my west-facing Phoenix AZ garage temp about 30 degrees in the summer. No lie, it was like 130 in there near the doors with the sun on it. Maybe even more of a drop. I would recommend it to anyone for both thermal insulation AND sound proofing.
Enjoy!