I put a piece of SMPTE RP-40/PA-35 test film into the gate and use that to align my enlarger.

It's all a matter of tinkering and fiddling to get it right. The pivot screw under the lens is where I make the adjustment.

First I put a level on the base board and ensure that is flat then I put a piece of glass in the stage and level that by aligning the carriage and rails. Then, finally, with the test film, I adjust the pivot screw until the test pattern is in focus, edge to edge and corner to corner.

It's not hard to do. It's just detail oriented and takes time.