The shape of the hook takes some tweaking and experience, but it must be shaped so that the lower leg is guided into the back of the loop. The back of the loop must be small enough so that the bird cannot slip its foot through the loop. If the metal used is high in copper, it is able to be tweaked, with your hand, according the age/size of the bird. If the bird's foot slips through, you've got to be able to squeeze the loop into a tighter, smaller hook. If the leg will not fit into the hook, it needs to be molded slightly larger. You get so you do this on the fly. Make an attempt, and then gauge whether you need it slightly larger or smaller, appropriate the bird you're after.
Hide the hook along your leg. Yes, they learn very, very quickly what that hook is for. Move slowly, keep them calm. You cannot make attempt after failed attempt. You'll get them so riled up that it harder and harder. Better to do this hooking in a confined space, such as a small pen. Many times, the bird will hop up on a nest box or lower roost. Bingo. That's the best "shot" you're ever gonna have. LOL
Squatting birds are easy. Just pick them up. Pick them sleeping off the roost? Easy. But, sometimes you've got to grab one in the middle of the day. Hope that helps.
It's just like tennis or a gold swing or hitting a 90 mph slider in baseball. Practice, practice, practice.