2nd as with any work you are doing, I suggest you save your original as a GIMP (.xcf) file to preserve layers etc.
3rd resize your pattern appropriately - I tend to work with 1024x1024 clothes files, so I chose a multiple of this. As it's for a skirt pattern I didn't make it too small.
4th save as a pattern file in your GIMP folder - mine (in W7) is "C:\Users\Admin\.gimp-2.6\patterns"
5th you get the option to put a name to your pattern
Enter the name and click save.
Once saved you need to reboot GIMP to make the pattern available to you
