Ed, if you know the brand and approximate year of bike you are lacing the chain guard and fender for, it may be helpful to first find the bicycle manufacturer catalog image for that bike so you can use that as reference as to how your particular bike should be laced originally. I did that for a bicycle of mine and it took a while to figure out the correct string pattern from the catalog image but eventually I got it correct.
I used a natural cotton twine from my local hardware store. If you look closely down where the fender attaches near the crank, you can see the inside, or bottom side of the fender. That should help a little.