Dressing a mannequin is not as easy as it may seem. We'll discuss how to dress the male mannequin. The female mannequin is about the same.
Steps
Decide the correct pose desired for the mannequin before starting.
Know that most mannequins have a fixed pose, so selecting the correct position is as important to the final effect as it is to providing a good fit for your garments.
Remember that if a shirt and/or pant is to be put on the mannequin, one of the arms/legs must be removed.
Leave one limb on, slip the garment over that limb and pull on completely. Then slip the loose limb back through the arm/leg hole and re-attach to the mannequin.