Making an icon set is very easy, given that you already have icons or know how to make them.
The icon set (*.sip) file is actually a ZIP file that contains icons and one text (*.txt) file, that tells the player how to use the icons. If you will take an existing icon set, e.g. “Crystal.sip”, rename it to “Crystal.zip” and unpack the archive's contents to the “Crystal” folder, you'll see this:
And here is how the “IconSet.ini” file looks like:
NOTE 1: you can assign one icon to several extensions
NOTE 2: you can add more extensions (associations) to the list, provided that Spider Player supports them
So simply by using your own icons instead of the ones provided and editing the “IconSet.ini” file, you can make a new icon set. After you finish, archive the icons and the “IconSet.ini” file back to ZIP, e.g. “MyIconSet.zip”. Then rename the archive to “MyIconSet.sip”. You are done!
NOTE: if you think you've made a cool icon set and want to share it with others, please email it (along with a 120×120 pixels thumbnail on white background) to firstname.lastname@example.org and it will be published on the website.