A pool noodle makes very good insulation and is easy to cut and work with. Choose your color wisely.
Step 1: Cut Pool Noodle to Size.
If you have a door that doesn't reach the floor, you may get drafts coming in under the door. Cut the length of a pool noodle to the width of your door. Then cut a channel halfway through the pool noodle the same width as the thickness of the door. I used a table saw to get a good straight line but a knife may work too.
Step 2: Slide Under the Door
Slide the pool noodle under the door to stop drafts.