Step 1: Adjust a Ceiling Tile
Step 2: Drill a Hole in the Mousetrap (optional)
Step 3: Position the Confetti Funnel
Since you may or may not be getting the funnel back ever, don't bother spending money on one. I just make one by curling a piece of paper into the right shape and trimming it down to size. You can use some of the rest of the paper to prop it up and keep it vertical, so the confetti will fall properly.
Once you've figured out the positioning, pull the card out. You're going to need it. Punch a hole in one end.
Step 4: Measure for Mousetrap String
First, figure out where the trigger string will enter the ceiling. This is a place where the positioning of the ceiling tile creates a gap for it, and is somewhere where the string can be relatively discreet. (You could even move another ceiling tile if nowhere on the one you started with is sufficiently concealable.)
For the funnel string, measure from the spot you just found to the funnel. Subtract the length of the mousetrap from the hole to the far end.
For the trigger string, just cut six or ten feet; you can trim it to length later.
Step 5: Position Mousetrap
Place the mousetrap on the ceiling tile, with the string dangling down in the spot you figured out for it earlier and the far end facing toward the funnel. Bring the card over to the funnel; you'll find it's most of a mousetrap-length shy of actually being able to block the funnel. If it's not, and can be placed under the funnel, then either your card is too long or you measured the funnel thread a little bit too long. (The measurement given earlier is inexact, since it ignores the length of the card and the knots.) Trim the card if it's too long, then take a bight of the thread of an appropriate length so the card just barely doesn't reach the funnel and tie a knot in it, shortening the thread.
Step 6: Set the Trigger
The classic place to stick the trigger is the back of a drawer. Not having drawers in the desk where I set this one up, I opted for a computer mouse, placing the mouse too far back to be very useful and tying the thread to its cord.
Now, set the mousetrap. With that done, you should be able to carefully slip the card under the bottom of the funnel (its thread does not need to be under tension in this state). Fill the funnel with confetti or glitter. Now, when the trigger is tugged, the mousetrap will be set off and that in turn will yank the card out from under the tunnel, dumping it on your victim's head.
Put the ceiling tile you used for access back in place to make your handiwork as inconspicuous as possible, then wait.