I've done it a couple ways. Best was making a simple trap using a panel of egg crate propped in a corner of the tank. I would either raise it up off the bottom a few inches or open a gap in one of the sides, and then repeatedly place food in that area. The fish are reluctant at first but eventually they all succumb to the temptation and go behind it to get the food, when the panel can be dropped to trap the fish. I trapped a horrible lemonpeel angel several times that way. After the first time you could tell he knew what was up but still could not help himself.
Many years ago I used elastic hair nets stretched across openings in the rockwork to catch a mean damsel. They become invisible under water and chasing the fish around the tank a bit got him caught quickly.