I just copied this from another forum, it is easy, you have an option have how you configure it. I Put the elbow on the standpipe then the valve then the tee connector.
The gate valve mod is very easy to build. The hardest part is locating a PVC gate valve.
In addition to the gate valve, you will also need an elbow, a tee, and some short lengths of pvc pipe.
The G2 and G3 skimmers both use 1.25 inch PVC and fittings. The G4, G4x, G5, and G6 skimmers all use 1.5 inch PVC and fittings. For the lengths of PVC, we are using: (1) three inch, (2) four inch, and (1) twelve inch.
The first thing you will need to do is remove the adjustable portion of the stand pipe that came with your skimmer (make sure you leave the bottom portion that is molded into the base of the skimmer). Use one of the four inch pieces to connect the gate valve to the bottom portion of the stand pipe. Use the three inch piece to connect the gate valve to the tee.
Use the second piece of four inch pipe to connect the tee to he elbow. Place the twelve inch section into the other side of the elbow. Slide the stub on the bottom side of the gate valve into the stand pipe on the skimmer. Test fit everyting and make sure this will fit into your installation, you may have to shorten or lengthen pieces of the pipe to match your needs.
We have bundled everything you see here in a Mod Kit. They are available here.
On the smaller skimmers, such as the G1x, G2, and G3, you may have to implement the Gate Valve Mod a little differently. This is because of the size the tee and gate valve take up. They end up being too high to effectively allow you to control the water level inside the skimmer. This is an alternate method that will work for the smaller skimmers.
Out of the skimmer exhaust, place your elbow, then connect it to the tee, out of the bottom of the tee you will install the gate valve, from the gate valve you will install a piece a lenght of PVC to empty into your sump.