We used a variation of the soda bottle trick when we were kids to catch minnows for fishing, it has served me fairly well since I got into reefing as well...
Cut the top off of a 2 liter soda bottle right about where it begins to narrow. Flip the top around and attach it back to the top of the bottle inverted like by poking a few holes around the perimeter and tying it back on with some fishing line. Add a line to pull the trap out of the tank with and viola, a fish trap. Some fish will swim right in, others will require some time to warm up to the idea, but usually, they will go in given time. Depending on the size of the fish you may need to trim some of the mouth of the bottle to allow a large enough hole for your target fish to swim through. Fish can swim right in, but have a difficult time finding the exit, especially when in a rush, giving you the time to pull the trap from the water.
A couple of other tricks with fish traps in general...
Some fish are more likely to enter the trap if it is suspended above the bottom rather than sitting right on the sand, also, some fish are more comfortable in one area of the tank than another, try to put it near where he hangs out.
You may notice that the fish you are after always pecks at the bottom or top of the fish trap trying to get the food he sees inside, angling the trap so the mouth faces somewhat up or down to accommodate your particular fish's preference can also help.
Try leaving the trap empty in the tank for a day or two, let the fish get used to the thing, they will be much more receptive to the idea of eating from it if it is not a strange new addition to the tank.
Good luck!
