With the Paterson grain focuser you simply focus the black line in the focuser by moving the moveable part of the focuser. Do this under the enlarger light at full aperture without a neg. There's a screw which enables the correct adjustment to be set.This adjusts the focuser for your eyesight. Put the neg into the carrier then the rest is done by the bellows adjuster on the enlarger to get the neg's grain in sharp focus. Do this also at full aperture. I'd take it beyond what you think is the sharpest grain focus and then move the bellows back into focus. Once you have done this on a couple of negs, it becomes second nature to do it correctly.
Finally adjust the lens aperture to expose the paper at a reasonable number of seconds, say between 5-10 secs. Unless your eyes are perfect a grain focuser gets your print into focus much better. Try focussing by eye then use the grain focuser. Unless you have an exceptional talent for focussing by eye I guarantee that the grain focuser will pull the grain into focus to an extent that the naked eye will fail to do.