In which order does the mantra go? This depends on the direction of wheel rotation. You want to have the mantra so that as the wheel spins it goes through in the order of Om Mani Padme Hum. If we think about a car the passenger side wheel spins clockwise. So the order should be Mani, the second and third symbol, to the left of Om, the first symbol, and so on. On the driver’s side the wheel spins counter-clockwise. So the order should be Mani to the right of Om and so on.
Does it matter which way the mantra spins? In essence as long as the mantra is spinning in the order according to it’s direction it does not matter if it is clockwise or counter-clockwise. A traditional prayer wheel is spun clockwise but if you were to look at it from the bottom it is spinning counter-clockwise it’s really about your point of view. According to Wikipedia some advanced Buddhists sometimes turn prayer wheels in a counter-clockwise direction.
The practitioner most often spins the wheel clockwise, for the direction the mantras are written is that of the movement of the sun across the sky. However, on rare occasions advanced Tantric practitioners such as Senge Dongma, the Lion-Faced Dakini spin prayer wheels counterclockwise to manifest a more wrathful protective energy. Before, during and after the practitioner turns the wheel, it is best to focus the mind and repeat the Om Mani Padme Hum mantra, as this increases the merit earned by the wheel’s use.