You can use any old string you have laying around, old rope, paracord, even shoe laces!
Place your cord/shoelace/string through your keyring so that on either side of the ring is only one piece of string/cord/shoelace
Pick the length for your key fob. Once you've decided how long the fob should be, make a loop from the left strand over the right.
Wrap the right side strand over the loose left end
Bring it around the back of your two strands and through the loop we made in step 1. This will create a loose knot.
Pull the knot tight.
Using the right strand of your cord, make a loop like before
Wrap the left strand around the loose end of your loop, around the back and through the loop on the right
Repeat steps 3-7 Until you reach the desired length.
Cut the extra string from your fob.
Burn the edges with a lighter to seal the fob together. You may, with a damp finger press the ends closer to the knots for a cleaner look.