Okie Dokie.
Before we get started I must say that the final product isn't brillant. The basic principals that I wll teach can be used to make MUCH better looking images.
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Step 1
Create a new document with the following dimensions.
Step 2
Select the paint bucket tool
Step 3
Fill the blank workspace, you should get something like this. Cool eh?
Step 4
Create a new layer (Keyboard Shortcut Shift + Ctrl + N)
OR
Press Layer > New > Layer
Step 5
Select the brush tool.
Step 6
Configure your brush,

Look for
this toolbar.

Click on the little drop down arrow next to the Brush: part of the toolbar.

You will get a screen like the one below.

Select the set of brushes you would like, then select the brush you want.
Step 7
Begin to brush in the opposite colour of the background (If it’s white… paint with black. If it’s black paint with white.)
After a few quick brush strokes you should get something like this.

Note: I used one of the brushes from Natural Brushes 2 and dragged it all over the image.
Step 8
Create a new layer and then switch to the opposite colour of the one you just used to brush over your base background. See step 4 if you don’t know how to create a new layer.
Step 9
Brush some more!