Illumination aside, two conditions need to be fulfilled. First, the negative, lens board, and base board must all be parallel. Second, the axis of the lens should pass through the center of the negative.(posted 9038 days ago)If you can get a commercial alignment tool like the little laser device, that's probably best. For a quick check, use a bubble level. Using cardboard shims, level the baseboard. Next, check the negative carrier resting surface. Easiest way is to find a piece of glass or mirror about 2" wide and 6" to 10" long. Put it where the carrier goes, then put the level on top of the glass. Check the lens in a similar way by holding the glass up to the filter ring of the lens. There are other methods, some of which are better than this, but this is easy and you probably already have a small level.
I haven't given any suggestions as to how to correct the alignment, as all enlargers differ. Look at how it's screwed together and apply some thought to the problem. Often the negative stage and lens are fixed to each other, and all you can do is move the head as a unit in relation to the base board. There may be adjustment screws. Or not. It may be possible to shim the column. Also, don't rule out something being bent, broken, or mis-manufacturered.
It's more difficult to check the lens and negative axis. I use a machined plastic plug that fits in the hole where the lens goes. This is raised up to the negative carrier, and the position of some concentric rings is noted (I can see the carrier by raising the condensers- not all enlargers will do this) A carefully made paper tube might also work. You'd like the lens to be within about 1/8" of center if possible. You'd also like the rest of the optical system to be on center, but there's little you can adjust there. Good Luck & Happy Y2K!