You will need: a unitard, fabric paints or markers, a friend or mannequin
I would recommend washing the unitard before you use it. Get fabric paints or markers in the colors that you want on your costume. You are going to need to paint or draw the colors on your unitard. There are three different ways you can do this. 1: Give your friend a pattern for your costume. While you are wearing the unitard, have him/her paint it. 2: Make sure your friend is about your size and shape. Have him/her wear the unitard while you paint it. 3: Get a mannequin that is about your size and shape and put the unitard on it. Paint your unitard while it is on the mannequin. I recommend this third one because it can be embarrassing when you're painting a person, especially when you paint certain parts.
Tips: Wear an old unitard under the one you are painting to ensure that you don’t get paint all over your body. Make sure someone is wearing the unitard when you paint it or it might look weird. Use a pattern or you might mess it up somehow. Make sure each color dries before you use the next color or they might mix. Use light colors before you use dark because the colors might smear and light colors don't smear so easily, unless you want to create some sort of effect.