You can make a clown costume by cutting up and recombining pieces of old clothes, preferably brightly colored ones with playful patterns, and then sewing or gluing on typical clown accessories, such as pompoms and oversized buttons. For example, you can make trousers with different-patterned legs. To make the characteristic giant bow tie, fold a piece of crepe paper, tie it with tape and cover the tape with more paper.
Complement the costume with a pair of oversized shoes and a colored wig. You could also add prank accessories such as a water-squirting flower from a toy store. If you don't have clothes with comical patterns, you can make spots with adhesive colored circles. For a child, you can use pieces of oversized clothing, such as an adult's tie. Pipe cleaners are colorful and can be shaped into various forms, such as flowers, and glued on.
It doesn't matter if the costume is poorly or unevenly sewn, as that is only in style for a clown. For the makeup, first apply moisturizer, then paint the entire face white with special face paint. Let it dry, and then paint details such as a red mouth and nose.
Another look is the "hobo" or tramp clown, for which you should combine odd pieces of ill-fitting, worn and old-fashioned clothing. The hobo uses brown and grey, rather than brightly colored clothes, and paints the mouth and nose red, but not the rest of the face white.