You need an image file that GIMP can open (PNG, GIF, etc.). And you need the command line tool ppmtowinicon from the netpbm-tools (on SuSE you have to have installed the package netpbm). Now perform the following actions:
- Open your image with GIMP.
- If your image is not a square then resize the canvas with GIMP.
- Scale the image to 16x16 (or 24x24) pixel (Image, Scale image).
- Choose File, save as.. and save as favicon.pnm or favicon.ppm with raw encoding.
- Close GIMP
- Convert favicon.pnm resp. favicon.ppm using the command line utility ppmtowinicon:
ppmtowinicon -output favicon.ico favicon.pnm
That's all. Now copy your favicon.ico to the document root of your website.