Well, it's fairly simple. You can open a font using the menu, or you can run 'swfedit <filename>'. Either way, once the file is loaded, you can select a character using the drop down box at the top, or using the up and down arrow keys. Then, click the left mouse button to draw a black pixel, or the right mouse button to draw a white pixel.

The controls at the right side of the window allow you to manually move the colored guide lines around. These lines don't affect the data in any way, and are only to help you make characters the same size : )