1) Make a new image and make it 6x6
(MaKe sure it's transparent)
2) Get the pencil tool and scale it to 1.
3) Draw a line diagonally top to bottom. Left or right.
4) Save it in the gimp folder into patterns.
5) Name it [insert name here].pat
You have to make the file .pat or it will not work.
6) Go on the patterns box and right click and click refresh.
7) You should see your line.
8) Now go to the bucket tool and make sure you click fill in pattern.
And tada! Sorry I cannot do pictures since I'm on the iPad. Hope it helps you