Hold the card against the bottom of the spool or wooden block with the pushpin or thumbtack sticking into the spool’s hole. The pushpin keeps the card from drifting off to the side.
Blow strongly through the hole in the top of the spool and let go of the card. If the card falls at first, experiment with different-sized cards or spools until you can make the card hang suspended beneath the spool.
Turn on the blower and direct it down through the hole. If you use a vacuum cleaner, be sure to use it as a blower, not suction. If you use a hair dryer, turn the heat off if you can, to avoid the dryer overheating.
Bring the plate up toward the hole from below. Contrary to what you might expect, as the plate approaches the hole it will be sucked up and held in place by the air blowing down (click to enlarge diagram below). The pushpin should keep the plate from drifting off to the side.