Firstly, make an image. Now make a new layer. You can
paint, place text or make an image on this new layer. The image
you've made will be the brush.
Second thing is to define the custom brush by going to
Edit menu and click the Define Brush caption. Next it will prompt
you to enter your custom brush name. Then select the paint brush
tool from the toolbar. Now you should see our custom brush in the
pallete. Click on it to make it active.
Now you're ready to
paint on the image using the custom brush you made.