Just to reiterate, the focus does snap very rapidly in/out of focus so go slowly or you will miss it. The aforementioned idea of putting the mirror of the finder over a high contrast edge (which, annoyingly, must also be close to the centre) helps you to not over-shoot the focus. Slow film and a low enlargement factor can make for very fine grain which is even easier to miss.

If you see an elliptical view through the eyepiece then you are too far away from the centre and need to move the finder more directly under the lens.

Good hunting!