Most staff learn a POS fastest by doing, not by reading. Here is a training flow that works.
Learn by doing
Run a fake busy service with test orders. Let each person take, fire and bill an order before guests arrive.
One cheat sheet
A single page of the common actions (open table, add item, apply discount, split, bill, share) beats a thick manual.
A buddy system
Pair new staff with someone confident for the first real shift. Questions get answered in seconds, not at the counter.