Loops of a bowstring are usually protected by thread wound around the loop, also known as serving. Sometimes it's beneficial to add thin leather protection on top of the serving. This can be done easily:

• Cut two thin but long slices of leather
• Puncture several pairs of small holes along their sides at regular intervals
• Sew the slices to the loops, starting from one end and sewing in spiral motion towards the other end and then returning back again, finally tying the ends together. This creates a nice series of "X"'s where the threads meet each other.

The results should look like this: