Lisa, a gate is a combination of two posts, the actual gate, hinges, locking mech, and "Z" supports.
Here is what you do: 1 determine the position of your gate, making sure that one of the existing posts is one of the two posts. Meaning, you can go on the left or right of an existing post.
2 now, determine the width of the gate, I imagine you would want about 30 inches. OK, put a paint sturring stick at the location in line with the fence, 30 inches from an existing post.
3 carefully remove the fence section (usually 8 foot, from one post to another. Lay this aside and take good care of it, it will go back and it will be your gate also.
4 Now, get your handy post hole digger and dig a hole where the stick is located and go about 18 to 24 inches down. Normally, I would not require you to move the fence away, but with a woman, you would bust your knuckles drying to dig a hole with the fence still intact, so this is the easiest way and safest.
5 Have your post ready, make sure its two feet taller than your fence, because you want about 2 feet in the ground. Now, you should have a bag of Fast Set Concrete handy. Fast Set will cure and set in about 2 hours. This way you can have the total project done in one day easily and never without security around your pool area.
6 set the post and make sure its level, use a bubble level to make sure. pour concrete all around the posts and fill it up. Sway the posts back and fourth to make sure all the air bubbles and spaces have settled in. Then level it again. Fill up the hole with concrete and make a hill of concrete on each side so the water sheds away.
7 Now, reinstall the fence back the way it was but do not nail the fence rails to the new post. Use only one nail per rail at this time. Do not drive all the way in so that you can remove easily with a claw hammer.
8 your fence is up. Ok, now, on the new post, install three heavy duty straping hinges for out door gates. Use two inch lag bolts and make sure all the bolts go into the three rails securely. Center the hinge pins in a straight up and down line even with the inside of the new posts. When they are all installed, you will go on the opposite side of the fence and saw the three rails off even with the hinge pin and walla, you have a hinged gate on one side. This para, is the one you might not understand or might have a problem with, if so come back for clarification.
9 Now, one side of the is hinged, now with your hammer, drive in two 16D galv nails into the post where the hinge is. This will not be the gate section but the section that is the stationary fence. You are nailing the fence back to the posts. Do this on the other end also.
10 now, you have the latch section to fix. Here is where you need to support the fence with the "Z" design. On the gate itself, you will have three rails going horiz and parallel. Here you want to install two new rails going from the top end of top rail to the middle end of the middle rail. This will be at an angle. Secure to the rail. Now do the same with the middle rail to the bottom rail. Now go to the other side and nail a nail into each slat going down the new angled rail. This makes sure the gate does not rack or angle down.
11 Now, when this is done, remove the nails of the gate section from the post. The gate now should open freely and swing. Now we want to install the latch.
12 the latch consists of a bar and a spring hand to catch the bar and secure the gate. Just follow the instructions on the package and you will have it.
Done. Difficulty level 6, tools post hole digger, level, something to mix concrete, nails, all galv, socket set, saw hand or circular, one 8 foot rail, hinge and gate hardware set, three beers. Time to install about 3 hours.