First, unfold your Pool-Pocket, carefully if shelled. Straighten the ropes out. You will see two side ropes and a large leader rope around the top opening. The leader rope should be pulled to form three loops, two on the sides and one in the back center.
If you are installing your Pool-Pocket on a flat rail, pull the leader rope (in the front center) thru, eliminating all three loops. Now you have two side ropes and one continuous leader rope.
Holding on to the pocket, toss over the side of the deck and position. Wrap side ropes around the top rail (they should be behind the leader rope), and knot.
The clove hitch is the simplest knot for this, though you may use any knot. Also good are the anchor hitch and of course the bowline. See Useful Knots.
Put pool toys in Pool-Pocket and pull leader rope taut and tie.
If you are hanging a 4'x8' pocket you will want to bring the leader rope "tails" up and around the top rail, in the center to prevent sagging. You may have to tweak it a little, like when hanging curtains in your home.