I bought aprons these last year at Michael's. I'll need a few more packs so I hope they still sell them!
The letters were purchased from Staples...
They are designed for indoor and outdoor use so they stick really well.
Position the letters where you want them...
a piece of tape helps to keep the lines straight.
Using a stencil brush, dry brush some paint around each letter. I used Martha Stewart's craft paint.
Be sure to pounce the brush and not brush it and keep the paint dry. You don't want too much paint to seep under the letters.
Now for the fun part...
after the paint dries carefully peel off each letter. Your letters will remain white.
These colors are child-like but this idea would also work with more sophisticated colors on a plain throw pillow, table runner, or monogrammed napkins.