First, we need a picture. I searched google and easily found a cloud picture that will work perfectly for this instructable.
Step 1: Selecting Unwanted Colors
Step 2: Removing Unwanted Colors
When removing all of the blue sky you will notice some spots needed to be touched up (removed) use your judgement and artistic license here to pic out the spots you don't want to show up on your new brush.
Once we're done with this step, we will set the picture to Difference mode, which will display the exact opposite colors that are being displayed, this will make it easier to remove and finalize the cloud.
Step 3: Finalizing and Saving Your Brush
When you save your brush, it automatically goes into your current brush selection. When you change your brush present, it will ask you if you want to save changes, click YES or you will lose your new brush.
Step 4: Finished!
You can do this with just about anything, some brushes take awhile to make depending on how detailed they are and how many colors they have. Also know that the larger you make your brushes initially, the better in the long run. You can decrease the size of the brush from big to small and maintain certain details, but you can't increase from small to big.