I'ved used a variety of methods depending on the fish.
I had to catch up my aro once to remove a anchor worm so I used a koi sock, he laid still perfectly, because it was pitch black and stillin water.
For a 16" TSN i used a triangle landing net with a string top so when you dislodge the base the 2 sides can be moved togther then i rolled him like THE TRUST said so they can't move.
For my oscar I tried nets but he was jus so damn powerful so i coaxed him into a bag.
For some spikey finned cats I use a new interpet net thats still stiff which there spines can't puncher, on older softer nets they get stuck.
And on rays i get them to swim into a tub, baring in mind all my current rays are under 10"