I am passing on links to the Gimp Talk forums on how to convert your brushes. These are for making brushes in sets, converting from Photoshop and other tips I may not include here. See post tags for saving and installing scm files if you don't know how to.
Brush Batch (scroll down)
Converting Photoshop Brushes to Gimp
(You may want to search the boards and browse the tuts there too.)
Now if you just want to make a single brush, here's how you do it:
You first have to answer a few questions -
How big do I need my brush?
Do I need different sizes of the same brush?
Do I need to rotate these brushes?
Do I need the brush made exactly like the same image of the same colors?
Or do I need the brush to pick up the current color selected in the palette?
To make a brush:
1. Open your selected image you want as a brush
Open a blank canvas with a transparent background in the size needed.
(Avatar brushes I recommend larger like 300px - see avatar tut for more info.)
Remember if you are using them for digital scrapbooking and it's an element brush you will want this large (500-1200 pixels), treat single letters and numbers the same way (for alphas if you are not making an alpha sheet template).
**The larger the brush and the more large brushes, the slower your Gimp will load - be sure to tuck these away in another file if you are not using them and keep a jpg preview of your brush in this same file so you can browse with windows or an image viewer instead of Gimp, since you may want to find these before you start Gimp.
2. With your image open-created/drawn-or added text/dingbat font, do any altering you need, usually you won't need to do anything.
3. If you want it in color as it is then skip this step. If you want it to pick the color of the color palette you need to change your image from RGB to Grayscale. Do this by clicking Image>Mode>Grayscale.
4. Save as brush one of two ways: a) File>Save As>"name.gbr", a pop up window will display, just click okay since these settings can be changed at the brush dialog box, b) Script Fu>Selection>To Brush. A. will save this file to a folder you choose whether it be the gimp brush folder or another, B. will save the brush into the .gimp>brushes folder automatically and refresh your brushes, with this image being your current selected brush.
Ideas for brushes:
copyright stamps with your screen name, site name, etc
"no piracy" stamp
elements like flowers, tabs, eyelets and slides
frequently typed words on your layouts or package previews
In the brush making process, occasionally some items do not convert over, like subtle lines. It really depends on the image. Also, colored items when switched to grayscale act differently when used as a brush. Black areas are 100% color and as the image goes gray, the opacity changes, with white being 0%, meaning there won't be any color in those areas. So, the areas that are gray will act as vellums, or semi-see through.