Step 1: Use your word processor
Step 2: Blacken around the letters
Step 3: Apply clear tape to the letters
Step 4: Add double sided tape to the back
Step 5: Cut the letters apart and apply
It works better to make the letters a bit smaller than the key surface. Notice the T. Smaller is better than an edge that extends beyond the key surface and continually catches on your fingers as you type. If a letter is poorly aligned after you apply it, peel it off and apply it again.
Refurbishing painted letters damaged by fingernail contact makes your keyboard easier to use, even though you know how to touch type and even though they are not a perfect replacement. They also seem to wear well and the touch of your fingers on them is nearly identical to a keyboard without the new additions to your keyboard.