Its basically the same as Windows or any other OS. All you really do is get an image editor such as Photoshop, Paint.net (not MsPaint!) and then download the default skin from http://www.minecraft.net. Then edit it with an image editor!
If you dont have photoshop then a good free alternative is gimp: http://www.gimp.org/
And then all you do is upload it to minecraft.net!