So to get infinite arrows you need to follow three steps:
- Get a companion (make sure they use a bow, Ex. Faendal, Lydia).
- Give the arrow that you would like to duplicate to your companion.
- Loot all the bodies that your companion killed or shot with arrows.
There should be around a 50/50% chance to be able to loot the arrows that has been shot at the target.
You should now slowly and steadily grow your amount of favourite arrows. Be sure to try to keep your companion alive.
There is a lot of more ways to get infinite arrows but this one I found the most easy (except for using the console, but I count that as cheating) to do for beginners.