What I have in mind is something like this:

Heres one way to do it:

Start by buying a 48" drywall square.I just picked up one for $ 12.99
Then. for example.. a 4 X 8' piece with one box cutout.
Measure out from one SIDEwall to the box.. lets say its 16 1/2" allow
"X"" for the box so the box cutout is: 16 1/2" ~ 20 1/2" "out"
I allow an extra 1/8" to 3/16" for "fitting" . I do the same with the
distance from the other ENDwall and get the measurement "down"
once you have the two sets of measurements, use the drywall square to mark
out the true position of the
box.. then I place a box over the marked area, draw a line around it and
cut.

If its an octagon box (essentially round) make the two measurements out to
the center point. Repeat on the board and cut out the circle.
"Measure twice, cut once" is a famous quote
Practice practice..you'll get better

Now if you had a sheet of cardboard from an old fridge box..you could
practice on that.

The pros now start to hang the board intact and make the cutouts in place
with a RotoZip.
I bought a "no name" Craftsman RotoZip clone at Sears for $ 59.00 and it has
a circle cutter,
side handle, and even a plunge router type housing that comes with it.