Find an image of G.I. Joe. Choose a picture of just his face if you can find one. This allows you to keep in step with the Pop Art conventions that Warhol established. Alternately, use an image not of the cartoon G.I. Joe, but of a G.I. Joe action figure. This choice plays up the playful aspects of Pop Art.
Convert your G.I. Joe image to black and white using Adobe Photoshop or other photo-editing software you have installed on your computer. If you do not currently have access to photo-editing software, use an online image editor.
Copy your entire image in the editing software you are using and paste another instance of it next to the first one. Paste two more instances of it below the first two. Many of Warhol's famous silkscreen prints feature this block-like arrangement of four or more images.
Open the converted image using photo-editing or paint software on your computer. Select two or three colors to use with the G.I. Joe image that appears in the top left corner. Match one color to the part of the image that appears black and the other to the part that appears white. Use the fill tool, usually represented by a bucket with a bucket of paint, to fill the black part of the image with your first color. Use the same tool to change the white portion of the image to your second color. If you select a third color, use it with any gray that appears or sprinkle it sparingly throughout both the black-and-white portions of the converted image. If you don't like the idea of creating your G.I. Joe Pop Art on the computer, print your converted black-and-white image using a high-quality laser printer and use paints or markers to add color to it by hand.
Repeat the above process with the other G.I. Joe images you pasted next to and below the first one. Make sure your color schemes are significantly different for each one.