We have three carriers, like this:

This is what I do:
1. Bring the carriers into the room about a day or so before I know I have to take one of them to the vet. (they scatter at the mere sight of the port-o-prisions)
2. Remove the top from the bottom of the carrier (the black knobs on the side move to "OPEN" and you can remove the top)
3. Grab cat and put into bottom of carrier and QUICKLY put top back on.
4. Snap prision back together.

This is the only way that works for us...even with the top loading carriers. They've learned to stick all four paws out or to climb up my head to get away from the carrier.

Good luck and let us know how it goes...