Im not sure if GIMP is downloadable on Mac but i know paint.net is. Type paint.net in google then find the web page then download it. As someone has posted the hat template you need to save it(you should know how to do that). You then go on paint.net then click open file on the top find the hat template then click on it you now have the hat template open! Now for the last step you click on new layer which is a white paper with a green "+" sign on it(it is located in the layers window). Grab the paintbrush and draw  . When your finished delete the hat template in the layers window and tada you have your hat! You then save the hat as a .png and when your your skilled enough you can make a .gif(animated hats)
hope it helped :p
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Hadi can you add me on game I have some more questions...