I happen to have the opinion, based on nothing whatsoever, that the palce to start is by having the enlarger in a plumb position so that gravity will be less likely to put it out of position and thereby reduce the frequency of need for realignment.
If the enlarger is in a plumb position and your easel is also, then you can first make sure that your negative carrier is moved to a plumb position..this is probably the most important point. Secondarily, you will need to make sure that the axis of your lens is perpendicular to the negative carrier..or that the front face is also moved to a plumb position.
There are a number of ways of doing this. There are a number of tools that can be used. The tool that I like is to use the type of level that has a bubble in the middle of a circle. Obtain a piece of 1/4 plate glass that is approx. 4 inches wide and 24 inches long. Carefully mount, I would suggest an adhesive, the bubble to one end of the 4" x 24" width. By having this piece of glass in the negative stage with the end that does not have the bubble attached and having the bubble level extended 2 feet outward one should be able to get the negative stage, assuming the enlarger is adjustable, quite plumb. Then do the same thing with the glass held against the face of the enlarging lens and again it can be made very plumb. Now you are done because you started by having the easel in a plumb position.
Two people working in concert on the effort will be very synergistic.