Make a drawing of what you want your sticker to look like. If you have a bit of artistic talent, you can draw the entire sticker design yourself, or if not, then just make simple a rough sketch of your sticker idea.
Take your sticker idea to a sticker design or sticker printing shop. Many of these companies may be around your local area, or you can look online for sticker printing companies such as Eman Printing.com or Printing Blue.com. Your initial design may have to be altered slightly for custom sticker printing. If so, the sticker printing designer will consult with you to ensure that you're satisfied with the sticker. If you've selected an online sticker printing source--depending on the company--you may be able to either customize your design using available clip art, have a sticker designed for you by a company designer, or pick from a selection of template stickers.
Select the sticker medium and shape that you'd like. Typically, you can pick from round, square, bumper sticker, or die-cut sticker shapes. You can also select the medium such as static cling, or peel and stick.
Pay for the services to have your stickers made and printed either online or at the print shop. Many online services will offer delivery of your purchase as well.