One way to do it:
Left hand counts decades.
Right hand counts aves.
Starting with the right hand (aves), press the index finger down to count one ave. Middle finger two aves. Ring finger for three aves. Pinky for four aves. At this point, all four fingers on your right hand are pressed down, but your thumb remains up. Five aves is represented by lifting your four fingers and pressing your thumb on the table. Six aves is represented by a thumb and an index finger on the table, and so on.
When you reach all your fingers and thumb pressed on your right hand, then you have completed nine aves. The next step is to press the index finger of your left hand. If you were just counting numbers, the number 10 is represented by only your left index finger being pressed.
Number eleven is your left index finger plus your right index finger.
Number twelve is your left index finger plus your right index and middle finger.
For the rosary, your tens are your decades. No left fingers is the first mystery. Left index finger is the tenth ave of the previous mystery, and represents the beginning of the next mystery, and so on. The last mystery happens when you lift all your left fingers, and press your left thumb down. You complete the rosary when you have reached the condition whereby only your left thumb is down, and all your right fingers and thumb are down.
So for each finger on your left hand, you cycle through the right hand in the manner I told you. Index finger, middle finger, ring finger, and then pinky finger. Lift all your right fingers and press down your thumb. Index finger down again, middle finger, ring finger, and then pinky. Then for your last ave, you press down your left index finger and lift your right fingers and right thumb.
It sounds complicated to relate in words, but if you just work it out for yourself, then you will see how easy it is.
Incidentally, you can use this method to do basic arithmetic really fast.