First, let's set out some terms to use. I have labelled the photo on the right, showing tent stabling with the flaps rolled up in order to show all of the supports, with two terms.
When I say 'vertical support', I am referring to the upright wooden or metal support at the edge of the tent that supports it from below.
When I say 'angled support', I am referring to the rope or woven strap that holds the tent taut.
When the flaps are hanging straight down, like in the drawing at left, they should hang between the vertical supports and the angled supports. If they are hung to the inside (towards the stalls) of the vertical supports, you will not be able to pull them out away from the stalls when it rains to keep the water away.
This drawing at right shows what you should do to the flaps when it's raining. If the flaps are not brand new, there should be small holes located along the bottom edge of the flap (the flaps don't come with them, so they have to be cut into new flaps unless there are loops of webbing in just the right place). Locate the hole closest to each angled strap and pass a piece of baling twine through that hole. Pull the flap out towards the angled support and use the baling twine to tie the flap to the support (if there is a loop near the bottom of the support strap, passing the baling twine through that loop will make it more secure by keeping it from slipping up the strap). Repeat for each angled support along the length of the flap.
Tying the flaps out allows the rainwater to run away from the stalls and it also keeps the flaps from flapping noisily against the stalls in the wind.