If the garage door isn't insulated, you could buy a sheet of insulation that looks like Styrofoam at a home improvement store and cut it to fit in the door panels. It is light and doesn't effect the opening of the door. I can't tell if you have an insulated door or not. I bought carpet at Lowe's called Kanga. It has a foam backing and is industrial carpet. It insulates the cement floor. I also hung a king size quilt over the door with clips that could be taken down if I needed to open the door. Painting the flake board could get pricey as you will need several coats, but for the first coats you could use mismatched paint or cheap paint from a discount store and then a coat of good paint. I sold the house I made the garage into a sewing room or I would post pictures, but it worked. You can also buy a screen that fits over a garage door opening if you want to open the door in good weather.