A) Hold the ninja by the head or upper torso to increase your chances of placing him upright successfully on a platform. If he falls on the tower/beam he will roll off so don't wait for him to try correct himself, just pick him up again.

B) If you have the towers you will probably have noticed that hovering the ninja in line with the tower will correct his depth on the screen so that he will land on the tower (similar to what happens with other apparatus). Look for the ninja's shadow on the platform to tell if he is above the platform. With the pyramid tower, the ninja will only be depth-corrected for the very top platform.

C) You can get the ninja to jump from tower/high beam to tower/high beam. But if you're only using one platform, it's quickest to get the ninja to jump towards you/the screen, catch him as soon as he jumps off, and place him back on the tower/high beam, then repeat.