Clean the door with warm water and mild detergent, such as liquid dish soap. Use a sponge, rag or clean sock to gently wipe down the door to remove any dirt or debris. The crack needs to be clean for the repair to last for any period of time.
Sand down the crack on a wooden or plastic door with fine sandpaper. If you don't have sandpaper in the house, use a scrubbing sponge or coarse pad. Steel wool will sand down the scratch but will severely scratch the door if you're not very careful.
Fill the crack. Wood putty is ideal for wooden doors. Otherwise, use toothpicks or pencil pieces in a wood door and seal it with a wood glue or epoxy. Standard white glue works temporarily but won't hold up for long. Plastic is best filled with an epoxy glue if you have it.
Sand the repair once it is fully dry with the same item you used earlier.
Paint over the repair to match the color as closely as possible to the surrounding surface if you have paint available.