I recently came across a wonderful tutorial for creating Mac OS X icons. However, that tutorial involves heavy Photoshop use, and I simply do not have Photoshop. However, I do have Gimp.app, a free alternative to Photoshop. Gimp isn't exactly the same as Photoshop, so following the tutorial was not a straightforward task. Therefore, I have created a tutorial of my own, which I hope helps others in my situation.

1. Make a new 128x128 image with a transparent background.
2. Add your drawings.
3. Drop shadows are nice, and can easily be added with the script-Fu menu.
4. Merge all Layers down until you only have one left.
5. Save this image as a png file.
6. Use the Layer menu, to select Mask->Add Layer Mask.
1. Select Layer's Alpha Channel and hit OK.
7. In the Layers floating palette, right click on the newly created mask and select Show Layer Mask.
8. Select all, and copy.
9. Create a new 128x128 image, and paste that mask.
10. Use the Layers menu to select Colors->Invert.
11. Save this new image as another png file.

12. Select the Thumbnail 32-bit.
13. Open your first png file (in preview will be fine), copy it, and paste it into the Iconographer window.
14. Now, select the Thumbnail 8-bit Mask.
15. Open the second png file, copy it, and again, paste it in Iconographer.
16. Select Save As from the file menu, give your icon a name, and make sure to select Mac OS X (.icns file) in the Icon Format pulldown menu.

17. Drag the new .icns file to the image well on the left side of the Pic2Icon window.
18. This will make the icon of your .icns file into the icon you just created, which makes it very easy to copy and paste onto other documents/folders/applications' icons (using the Finder's Get Info).

That's it. Using this method, you can create great looking icons in a matter of minutes. Great for programmers, artists on a budget, or those who want to replace an ugly icon with one of their own.