You don't mention which enlarger you have.
My experience is that Beselers are the easiest to align, Omegas the hardest.
In fairness, Omegas seem to hold their alignment better, but since most enlargers I know of have been moved (some multiple times) the ability to align quickly and easily is paramount.
You will need an adjustable level, which can get to a level from any surface, like a baseboard since the horizontal position of the level bubble itself can be adjusted, CPM Delta or Saunders make these items.
Having done these for a lot of high schools, I am speaking from experience...some good, some bad!
Note that the aperture used on your enlarging lens will also have some effect on how much of the image on the easel can/will be in focus, too, just like with a camera lens, if your whole system is not parallel.