This is a technique for boosting your Sneak and Archery skills very easily and very early in the game. You can do it as soon as you pick up your first follower, so potentially after your first visit to Riverwood only five or ten minutes after the start of the game. You can do this if you want to quickly become a master stealthy archer-assassin and play the rest of Skyrim that way.
All you need to do this is a bow and plenty of arrows, a follower, and an isolated wide open space. I’ll describe the way to do this as early as possible after starting the game. Go to Riverwood after escaping the dragon attack on Helgen Keep. By this time you should have picked up a bow and some arrows, but buy plenty of arrows in the town. Sell some of the stuff you’ve picked up that you don’t need if you have to. Then pick up a follower by doing the quest where you have to deliver a letter to someone, which takes about one minute.
NOTE: If you don’t want to do that quest because you consider it somewhat devious and unbecoming the type of character you want to play Skyrim as, then you can wait until you get your first follower in Whiterun. It should only be a few hours of gameplay time away.
Now take your follower and your bow and arrows out into the countryside next to Riverwood. Instruct them to wait somewhere, then move away from them until they can no longer see you. Now enter crouch/sneak mode, and move back to within shooting distance of them while making sure to remain undetected. Now shoot arrows at them continuously while remaining in sneak mode and undetected. They won’t move, they won’t die, and they won’t retaliate against you. Watch your Sneak and Archery levels rise rapidly. Your character level will also go up the more you increase those skills.
Do this for as long as you want or for as long as you have arrows.