Basically setup and load the tubes with shells; lots of attention is given to safety buffers, and things like orienting the racks so they can't fall facing the audience.

Some states require electronic firing, so everything gets a squib tied into the fuse, other places you can hand-fire with a road flare, which is more reliable, but dangerous. Anything that doesn't launch needs to be re-squibbed or extracted.

Its also wicked exciting, and fireworks from directly below look entirely different. My first show hand firing 6" shells, I distinctly recall knee jerk yelling "what the fuck" over and over, grinning ear to ear; you can really feel the pressure wave from the launch. It also paid $10/hr

There is also an amateur fireworks association as a fyi; people still hand build shells :)

Edit: as for timing, they'd have patterns like, x3 3-inch shells, x2 6-inch, and 1 8-inch; color and pattern were intentional choices. For hand fire, they'd yell out the shell size to fire, one person per size