As always, I start with a 12x12 document at 300 dpi. Grab your type tool and your favorite outline font. I used Arezzo Outline. Go ahead and type in the letter you want to customize and right click on letter layer and choose rasterize type.
You will get a marching ants outline around the inside of your letter. For the first way you can change it up, let's make a scribbly pattern inside the letter. So, grab your brush tool and choose a hard round brush with size of about 10-15. Choose the color you like, I chose green, and start scribbling on your letter and you will see the scribbles stay inside your letter instead of going all over!
Another thing you can do is just select the inside with your wand tool and fill the inside with a solid color, like I did with the letter B.
From here you can keep using your brush tool and doodle inside, draw some dots, stripes, the possibilities are endless! For the C, I just used the same hard round brush as I did for the scribbles but just stamped down once here and there to get a random dot pattern.
For the letter D, I grabbed a patterned paper, shrunk it down, and then selected my letter and clipped it down (hover your mouse between the layer you are clipping to and the layer with the item you wanted clipped and then hit your alt key until you see two intertwined circles and then click your mouse once), which changed the outline of the letter to the patterned paper! It is all so easy and fun and you can customize it to go with aboslutely anything!
Customizing fonts is fun and easy and will stretch those fonts and alphas you have in your stash! Have fun! As always, if you have any questions, feel free to contact me at firstname.lastname@example.org.