Quick OS X Icon Tutorial

Mac OS X Custom Folder Icon-Making Tutorial

To begin with, this tutorial assumes you are using Adobe Photoshop and Mscape Iconographer X v2.2 and are familiar with the operation of a computer and the basics of Photoshop. If you are an advanced user (or you're impatient with tutorials), this basic process should be helpful. That said, here is the complete step-by-step of how to make an OS X custom folder icon using Photoshop (almost any version will work) and Iconographer X 2.2:

Start up Photoshop and Iconographer
In Iconographer, go "File > Open" and find the folder who's icon you want to edit and open it.
Make sure the "thumbnail 32-bit icon" member is selected in the member panel
Press Apple-A and the Apple-C to select all and copy.
Switch to Photoshop, make a new document, and press Apple-V.
While in Photoshop, either open the image that you wish to put on top of the blue folder pic, or draw directly onto it. Don't go outside the outline of the blue folder unless you know what you're doing, because making a new mask (and that's what you'll have to do) is beyond the scope of this tutorial.
Save your Photoshop image now if you want to preserve the layers for later use.
Flatten your new Photoshop image.
Once again, select all and copy it.
Back in Iconographer, the "thumbnail 32-bit icon" member should still be selected. Press Apple-V.
Still in Iconographer, choose the next member down; it should be the mask. Select it and copy it.
In Photoshop, create a new document and press Apple-V.
Resize that document to 48x48 pixels. (Image > Image Size...)
Select it and copy it.
Switch back to Iconographer, and scroll the members window a little until you see the "Huge 8-Bit Mask" member. Click that member and press Apple-V.
Switch back to Photoshop yet again and your custom icon. Resize it, too, to 48x48 pixels and select all and copy it.
In Iconographer, click on the "Huge 32-bit Icon" member and hit paste. Click on the member right below that one and hit paste again.
Switch back to Photoshop. Resize your icon to 32x32 pixels and paste it into the members "Large 32-bit Icon" and "Large 8-bit Icon."
Last time now... resize your icon to 16x16 pixels and paste it into "Small 32-bit Icon" and "Small 8-bit Icon" members.
Now to put that icon on your folder! I recommend doing a "save as" first, just for your files. In fact just do it anyway because I'm not entirely certain how saving works in Iconographer, and I know this will work.
Now that you've filed away your new icon somewhere, choose File > Save Into File... and navigate to your target folder. If it tells you that any masks need to be regenerated, just click "Regenerate".
Make absolutely certain that the icon format is "Mac OS New (Resources)" !!! If it isn't, pull down the format menu and choose the correct format.
Hit Save! You're done!

The quick description of how to make a custom X folder icon:

Using your icon-making program, get the basic OS X folder icon at full size (128x128 pixels), and paste it into your image-editing program.
Using your image-editing program, make your new folder icon
Paste your new image back into your icon program.
Make three other sizes of your image: 48x48, 32x32, and 16x16. (If your icon program automatically does all the other sizes, then you might not have to do this.)
Paste those images into the appropriate places in your icon program. You don't really need to worry about any of the other possible sizes.
As far as the mask goes, unless you're so enterprising that you absolutely have to stick a picture onto the folder that goes off the edges, you should just be able to keep the basic folder mask. If you're using Iconographer, you may find you have to paste the mask into your image-editor and resize it to at least the 48x48 size and pop that one back into Iconographer.