I could summarize it for you - you need some straight pipe, a T , a gatevalve and possibly a coupler in the same diameter as the output tube.
1. cut a 2 pieces of pvc pipe 3-4" in length you
2. pull the current riser off the output to the skimmer
3. attach the T to the verticle riser so that the T is installed like this |-
4. attach the gatevalve to the output of the T using a piece of cute pipe like this |--(gatevalve here)
5. add a riser to the top of the T to keep water from splashing out.
6. tune
If anyone is with more clue - feel free to correct me. I've only done it once, with Mark (Reef55)'s help.
You could also set it up as illustrated above by Casino. Although I think the top of that drain pipe is too high.
It's the same principle - you are looking to restrict flow on the output and have the drain under or near water to minimize splashing.