First of all, open up the image you want for a brush. Erase all the background if you like, but if the background is plain white, then you don't need to erase. After, go to "Edit > Define Brush Preset" as shown below in the screenshots. Then, a box will pop up and you can name you brush. I named my brush "L".
After, click the "down arrow" thing where the brushes are and then click the arrow that looks like this: ">". Look for "Preset Manager". Look at the screenshot below.
Then, select the brush you just named and make sure you click on the image. Click "Save Set" and name the set what you want it to be.
After, you may want to close out Photoshop since the brush set won't appear in your dropdown selections. Open it back up and then select your new brush set. If you would like to add more brushes to it, refer to the first two steps. AFTER, you go to that dropdown brush selections and look for "Save brush set".
There, you have yourself a brush set! If you want to make your brush a download, just go to Winzip > Create new .zip file > Name file > Place the .abr files in the .zip file > Finish.
If you have any questions/errors with this tutorials, please use the contact form to contact me. Hope this tutorial came in handy!