Only recently I have been looking into how to improve the use of Sourcefourge's SEASHORE for Mac OS X. The following is a step by step guide on how to create custom brushes.

1. Right click on SEASHORE in FINDER and choose the option "Show Package Contents." Click on the "Contents" folder, then "Resources," then "Brushes." Create a Folder entitled Custom Brushes, or My Brushes.

2. Open SEASHORE and create the template for your brush.Make sure Alpha Channel is on.
(ADVANCED: create this template with any color other than pure black (Not counting partly transparent black) and no matter what color is selected for your Foreground you will paint it in the original colors of the Brush.)

3. In Spotlight search for BRUSHED. Open the application. Click on >File>New.

4. Go back to your SEASHORE document and select only the pattern you want to turn into a brush.

5. Paste this into your BRUSHED document. The slider labeled spacing works on a percentage value. When the brush has been slid the provided percentage of its size it will place down another of this pattern. Slide the slider to what you think is right for your brush.

6. Name the brush by using the text box.

7. Click where it says full color to change it to monochrome. Leave this to full color for colored images that you are turning into brushes. Save the brush, and put it in the folder you created in step (1).

8. Restart SEASHORE and the brush will be under the tab Brushes>My Brushes/Custom Brushes in the Options toolbox.