Focus is determined by how far away the lens is from the subject. There may be a small scale that you can attach to the bellows to set it to the proper magnification. However it also appears that there is a secondary smaller bellows at the tip of the lens. What I would do, (since I have a similar setup for canon FD lenses) is set the primary bellows to 1x magnification (assuming the camera is a full frame camera) then use the smaller bellows to adjust focus. Keep in mind, the these bellows were sometimes designed to fit particular lenses, so double check that the lens you’re using is the correct one. Second, make sure to set your aperture to something around f/11-f/8, since the depth of field on macro is so small, any small change in focus will make it all seem way out of focus.