We now need to mount it to the deck. I used only three clamps: one close to the edge, and two at the very back of the boom pole.
Mark off holes, and drill them out using a wood bit smaller than the screws.
For the bracket near the edge, I ran into a problem. It appears that the plate that holds up the railing post was directly under the boom! So the hole on the right couldn't be drilled.
I switched to a metal bit and drilled it out. No problem!