If you don't have experience, you start by
looking at the engine manual, and asking
others with experience. You get several
propellers in the range that is recommended.
You try each propeller on the bench and check
the engine rpm. Of the ones in the correct
rpm range, pick one that gives the plane a
good tug at full throttle while you hold it. That
one will probably get the plane off the ground
ok. Then after a couple of flights try one of the
others. If the plane needs full throttle all the time
to keep up flying speed, maybe a little more pitch
and less diameter. If it seems like a rocket try more
diameter. If it doesn't climb well try less pitch.
There should be at least 4 propellers of various
diameters and pitch that will fly the plane acceptably.
One of them will be the best for what you want to do.
Jenny