[johnnyrod]

You need to use the aperture wide open for smallest depth of field. Most people seem to collimate at infinity either direct or by back-focusing, but for me it has too many pitfalls, not least of which is even the minimal DoF at f2.8 is quite sizeable at infinity. So I do it at about 5ft/1.5m as the DoF is very shallow and you can get the focus spot on because of that. I use a spare split image screen from an SLR camera, but you would be fine with a piece of ground glass or similar. Stick it on the film plane and examine it with a magnifying glass. Adjust the distance to be exactly 5ft/1.5m depending on your focus scale, measured from the film plane - some cameras have a mark on the top housing indicating where this is. Use a subject that is flat and high contrast - I use a monochrome text poster on my laptop screen. It also makes it easy to adjust the rangefinder at the same time.

For 120, the focus screen is too small so I've made a bit of cardboard to attach it to. Cut a piece the size of the film. Cut a large hole in it. Cut another piece of card larger than the hole, with a smaller hole in it, and stick them together. Stick the focus screen to this so it sits inside the larger hole (i.e. that side of the smaller piece), now it will be in the same plane as the film as you've cancelled out the thickness of the cardboard.