These things are pretty simple. They operate just like an old folding camera.
Remove the back. Which side is the wind knob on? That's where the empty spool goes. Probably there is a knob on each side that lifts up to retract a pin that holds the film spool in place. The fresh roll of film goes on the opposite side. The film goes from one side to the other with the black part of the paper backing toward the lens. Thread the film and wind it maybe one turn and replace the back.
How do you know which window to use? One should show numbers from 1-8 for 6x9 and one should show numbers 1, 3, 5, 7, 9, 11 for 6x12 (you're using the numbers for 6x6, but you're going to advance the film to every other frame for 6x12).
How do you focus for medium format on a 4x5" camera? If your groundglass isn't already marked for 6x9 and 6x12, measure the film gate on the rollfilm holder to determine the actual dimensions of the frame and you can mark the frame lines on the groundglass in pencil. Focus normally, but you'll have to remove the groundglass and clip in the rollfilm holder when you're ready to take the picture.
To take the picture, just remove the darkslide and shoot as if you were using a regular filmholder. Replace the darkslide and advance the film after the shot to avoid double exposures.
When you get to the last frame, wind the film all the way and change the roll.