Extract into your Skyrim\Data folder
Start the launcher and click "Data Files"
Put a check mark next to GetArrowsBack.esp (if it's not already checked off)
Launch the game and enjoy!
Update: Added "Hunter's Discipline" Perk mod so it also gives you all your arrows back.
Update: Per request added a version to get 80% of your arrows back instead of 100%
Use the main file if you don't plan on getting the Hunter's Discipline perk. If you have the perk, use the new mod file. You don't need both.