Ok this is easy as can be. I have done this a few times for myself and a few other people. what you do is the following
Step one: Get a 20oz plastic bottle
Step two: Cut off the top of the bottle right before it starts to tapper. (Basicly cut it off at the place on a pop bottle were the lable starts.)
Step Three: Cut off the part of the bottle you put your mouth on so its big enough for the fish to swim into.
Step Four: Put flip the top of the bottle over that you just cut off and put it into the bottle.
Step Five: Poke 3 holes in and use Fishing line and tie it with fishing line so its secure.
Step Six: Take the fishing line and cut 2 long peices and tie them to either side (if you only do one when you pull the bottle up it will cause the bottle to turn upside and loose the fish)
Step Seven: Put some food in it and Place it in the tank
Steph Eight: Sit and watch, The damsel will swim into it and it will have a hard time finding the escape route especially when nervous because it will start swimming randomly and rarly do fish try to escape UP (the reasoning for line being on both sides) They want to dart to the bottom and hide in the rocks.
This works really well and should be sucessfull in about 20mins and will allow you to not destroy your tank.
Heres some examples
Catching Your Fish - DIY Fish Trap