Choose your design or design theme. It's important to plan out what you want to achieve with your poster. Will it be a brightly colored sign, mathematical formulas, the solar system or abstract?
Choose the media you want to work with. Do you want to paint? If so, do you want to use watercolor or acrylics? Do you want to use charcoal for a black and white design? Do you want to use markers or colored pencils? Consider what medium will facilitate the best look for your design. Go to the art store and gather your materials. Tell the salesperson what medium you are working with and they will tell you all the supplies you need.
Measure the length and width of your door with the measuring tape. Go to an office or art-supply store with the measurements and buy an art paper roll with which you can cut a piece the size of your door, either the same width or slightly wider.
Cut the size of your door out of the paper roll. Use the length and width measurements of the door to do this. To practice, cut two pieces of the paper that are door size, or use a smaller piece of paper from the artist's roll. If necessary, measure the exact location of the doorknob and cut a hole at that location on the paper. Be sure to do so before placing any artwork on the paper.
Make your image on the paper that you cut from the roll. If you are drawing a detailed image, draw it in pencil first and then go over your lines with your medium.
Attach the poster to your door using double sided tape. There should be a piece of tape at each corner of the back side of your poster and at least two pieces on the top and bottom of the back side.