Im not sure if paint.net is downloadable on Mac but i know gimp is. Type gimp 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 gimp 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 (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