Flat and sturdy is the starting point.
It is hard to build straight if the table is twisted. How you accomplish that is a matter of preferences and possibilities.

Then you probably need a way to fix your parts over the plan without ruining the plan. Again, there are a few ways to do that.

You might need a way to make holes for fixing things to the plane (engine, servos,...) or letting things pass through parts (exits for pushrods maybe). I do not think anybody mentioned a drill. You are not working with heavy materials but you need to be pretty accurate, so the drill need not be powerful, maybe not even with electrical power.