In GIMP there are subtle differences from the Photoshop tutorial available in the userbars.org mainpage, so I decided to make this little tutorial. Hope u like.
first we make a nice new little 6x6 image with a transparent background

now zoom it to 800%, pick a drawing tool (the pencil is ideal to use on hardpixels) and create a pattern (I chose to do an X, but to follow true to the userbar trend the diagonal line is the de facto standard, and in all honesty is looks better then most other patters)

after your happy with your pattern it's time to save it as one:
on the
save as menu click
on select file type and pick
gimp pattern name it and save it.
GIMP will ask again for a name of the pattern (this name will show up when you hoover over the pattern in the pattern selection menu)

Ok, now the only tricky bit. we have to move the newly created file to gimp's pattern folder.
Using your file browser copy the file (extension .pat) and paste it into the correct folder. where is it?
if you are using Linux it's most likely in your home folder under
.gimp-2.2 and then
patterns
side note: yes, I did manage to misspell "squared" half a dozen times.
if you can't find the .gimp-2.2 folder it's because it's hidden. Using Gnome/Galeon press CTRL+H wile browsing you home folder and all hidden folders will be visible. for KDE users... I have no idea so if anyone knows tell me and I'll later edit the post.
if you are using windows its in C:\Documents and Settings\USER_NAME\.gimp-2.2\patterns
(no screenshot, sorry)
all done! if you now open the pattern dialog box make sure you press the "reload" button so it searches for new added patterns and it's right there waiting to be another tool in your GIMP arsenal.
Hope it helped.
oh, and that cross pattern is terrible.