Simplest is probably to use a dark curtain material (two layers if you can not get totally light-proof material). Use a strip of Velcro above the door and at a few points down the sides and sew the other half of the Velcro to the material. Make the material 10cm or more longer than necessary so it seals the bottom of the door. Make the material overlap the door edges by as much as possible to help keep the light out.
Good luck, Bob.
P.S. Your English is fine!