If it's the larsen trap with one large compartment with 2 adjoining (not sure if the name is used with other styles)
you don't use the large bit till you have the decoy,
ours has 2 'gates' on the top of the 2 small sections, make a nest in the bottom with eggs as bait.
we use 2 half sticks which prop the gate open across the bottom. when the crow lands on the stick it gives in the middle, crow drops to the floor while gate flips shut. crow caught, first crow can than be transfered to bigger compartment, (use thick gloves!), where there should be a perch, food and water. sometimes better if you bring a crow in from a different area. some people clip wing feathers in case they get out.
just been talking to gamekeeper today, he just uses one stick but it holds the gate open slightly differently.
You have to be prepared to kill them once caught, either shoot with air rifle or neck them, some people think it's OK to let them go some miles away!
Hope this gives you some idea, sorry I'm not better at explaining
We've had 4 magpies in as many days, hopefully stop them multiplying before they start!