Install it after glassing. Additionaly I'd suggest drilling a 1/32" hole in the center of the plate, velcro a painters sponge on the bottom of the inside of the hull under the plate and you won't need a breathing tube. I've paddled in very splashy conditions and never needed to squeeze any measurable amount of water out of my sponge.  SEEYA Jack

Additionaly, I should have said after glassing and varnishing. In other words, the last thing you do to the deck. SEEYA

I just started building mine a couple days ago.   What is a deck plate?

Check out http://www.clcboats.com/shop/products/boat-building-supplies-epoxy-fiberglass-plywood/boat-fasteners-hardware/deck-plates.html. Because your board will be hollow and air tight you'll need a way to address sudden pressure changes due to changes in ambient air temperature inside and outside the hull, thus the breather hole / tube called for in the plans. Some builders think these tubes are not aesthetically pleasing and try other approaches. My approach was the install a deck plate in the deck to 1: give me access to the interior of the hull for storage. I built a 8" x 8" box under the plate and 2: I drilled a 1/32" hole in the center of the plate to relieve the air pressure. I live in a hot area (Miami Beach) and this tiny hole has proved sufficient and no water has entered. Enjoy you build and paddling. SEEYA Jack