Step 1, make sure that you can focus on the boundary of the central focus spot with your finder. Use the flip-up magnifier on the waist level finder. I am assuming that you are using a microprism screen. If the flip-up finder won't let you do it, take the finder off and use a loupe.

2. focus the camera so that both the focus spot boundary and the image are in sharp focus. Use something with clear edges for the test.

It does help to keep stray light out of the finder. If you cannot get the focus aid edge sharp, then you need to adjust your eye correction somehow. Either with the finder or with glasses.

You can do a sanity check with a piece of translucent tape taut across the film guides with the back open. Lock the shutter open on B with a cable release and verify the focus of both lenses.

It is not uncommon to rack lenses back and forth to find the peak of focus, but you should be convinced that there was a point of focus as you go past it.