If this is one of those flat flies that you can install either way:
without any springs installed, put the sear in the plate with its screw. No bridle needed at this point. Then put the fly in the tumbler and set assembly in the lock plate.
Put a little pressure on the sear tail, and slowly rotate the tumbler back, like it's being cocked. The fly should be under the sear nose until the tumbler gets to full cock position, when it should pop out ahead of the sear nose. Rotate the tumbler slowly to the down position, and the fly will make the sear nose ride up over the half cock notch.
If the fly does not operate properly, flip it over and run the test again.