Where should I stand?

Directly behind the stumps at the bowler's end, a pace or two back, so you can see the bowler's front foot land at the popping crease and still look straight down the pitch to the striker's stumps.

The bowler runs past you on one side. Ask each new bowler which arm they bowl with and whether they're going over or around the wicket, so you know which side to leave clear.

Once the ball is played, move sideways to get a clear view of the crease at your end for any run out, without getting in the way of a throw.