The inner three terminals are A, B & C. Which one you call "A" is arbitrary. Put a mark with a sharpie next to the terminal you want to call A. We will call the outer terminals A1, A2, A3, A4, B1, B2 and so on. Makes more sense than calling them 1 thru 12. Set your DMM for BEEP if it has that setting, otherwise set it for the lowest Ohms range.
1) Rotate the switch full CCW (as viewed from the knob side).
2) Connect one lead of the DMM to A.
3) Touch the other DMM lead to the outer terminals near A until you get a BEEP or a resistance reading near zero. The terminal that beeps (or reads zero) is terminal A1. Put a mark with a sharpie next to terminal A1.
4) Rotate the switch one click CW and repeat step 3. The terminal that beeps is A2.
5) Repeat step 4 for terminals A3 & A4.
6) Rotate the switch full CCW.
7) Move the DMM lead from A to the next inside terminal, we'll call that one "B."
8) Perform steps 3 - 5 to find terminals B1 - B4, but don't make any marks.
9) Repeat steps 6 - 8 to find terminals C1 - C4.
Make sense?