You'll want to keep all elements of this surprise reveal a secret from the audience--not the performers. Place the confetti cannon in a central location out of sight and mask its position with scenery. Keep the tip of the launch tube clear so that nothing obscures the opening. Be sure it is far enough down stage to launch the confetti into the air and land where you intend. You will want to test this with cheap white confetti. Tests are messy and an extra cost for confetti and propellant, but they help assure the desired effect. But first lets get the cannon set up.
There are a variety of confetti cannons on the market. Firing mechanisms and launch methods also vary. There are manually operated mechanical and electronic remote systems. There are systems that use CO2 cartridges, air compressors or pyrotechnic charges. For the purposes of this piece we'll be assuming the use of a cannon that uses compressed air and a electronic solenoid activated release valve for the firing mechanism. Set up the remote system as per the unit's instructions. Plug the cannon into an electrical outlet. Set the release valve to the off position. Pressurize the air tank. Load the confetti into the launch tube and tape a paper percussion cap over the opening.
Hang the black light fixture or fixtures out of sight in a position well in front of the cannon. Remember, you want the confetti lit from the audience, so hanging the fixtures overhead in the house is a good idea. Plug the fixtures into their own circuit that will not be affected by an auditorium blackout. This is done for two reasons. First, many UV lamps require warm up, therefore they must be on well before you perform the reveal. And second, you will be killing all other lights for this effect, but want these to remain on.
Now we'll run a test. Let's assume that you will present this effect as the finale in a musical number. The reveal will take place on the number's closing beat. Therefore, for maximum impact, the auditorium will have to be blacked out prior to the reveal. The sequence for the effect is thus:
1) Final few notes of the musical number play.
2) Blackout (on a beat). Brief pause (preferably on a rest).
3) On the final note of the musical number the firing button is pressed and the confetti is launched. The auditorium remains dark as the confetti falls.
Now clean up and repeat steps 1 through 3 in prep for your show.
Resources:
Confetti cannons come in all shapes and sizes and price ranges. They can be purchased or rented. Many lighting rental businesses rent them. They can also order the confetti product you require or you can find it on the Web. These are prepackaged into sleeves that slide into the cannon's launch tube. You can use flutter-fetti or streamers. Be sure to order a nice florescent color mix.