1. Make sure the 35mm canister fits sturdily in the place the 120 film normally is. You can use foam or something else to make it fit.
2. Pick up the 120 take-up spool and tape the sides until you have an opening small enough to fit the 35mm film without it able to move up and down.
3. Tape off the red window with some isolation tape or a piece of aluminum foil, this is because 35mm film has no paper backing, therefore leaving the red window open only causes fog on the film.
4. Now go out and shoot like you normally would, note that you have to “blindly” wind as you cannot see the numbers behind the red window. Normally a full turn of the Clack’s advance knob is enough, although this varies on every camera, just wind some more if you are not sure.
5. This is the most important step, when you finished the entire 35mm roll (the advance knob will simply block, dont try to wind further as you may break the film) go to a complete dark room (or a dimly lit room and hide under a thick blanket) and manually wind all the film back in the canister.
6. You can now let the film be developed at the lab, make sure they don’t cut the film, as they will cut right through the 9cm long negatives, I learned this the hard way.