That guide might work best for making spell icons, but you really don't have to do all that just for a macro icon.
The first thing you will want to do is to make a new folder in your "\World of Warcraft\Interface" folder, and call it "Icons". Now fire up your graphics program, and make the image you want to use. I saved mine at 64pixels x 64pixels, 8bits per channel (24bits) without an alpha channel, and as an uncompressed .tga. I started with a 256 x 256 image so I could work with it a little better (bad eyes) and re-sized it after I had it the way I wanted it and had flattened (merged) it to a single layer.
Here are two I did, and an in-game pic:
After you have it in your "Icons" folder, and you get into the game, open your macro editor. Your new icon should be there, but
be aware that you are going to have to hunt for it. It won't automatically show up as the last icon in the bunch. Mine showed up almost half way through. I've put 6 icons in there, and luckily they've all shown up together.
Hope this helps you. Good luck, and Have Fun!