The best way to align an enlarger is one of those laser devices, but since you probably don't have access to one, there's a trick you can do with two mirrors. Never tried it myself but a web search should turn up an explanation. I think you get two mirrors, scratch a tiny bit of the backing off one and stick that in your neg carrier, lay the other bit on the baseboard and then make adjustments to the enlarger head until the beam of light passing thru the top mirror reflects back on itself. Something like that. The problem with using a level is how do you get it to show the neg holders relative levelness. Depending on the design of the enlarger, this might be possible, might not. Another think you can try is to get and old bit of clear film and draw a cross (and possibly some more lines) on it with a pen. Stick that in the enlarger and focus. Check that the lines are sharp all over, make adjustments until you're happy