Each of the legs has labeling on it to indicate where it should be attached. And there are three different size legs. Here's the sizing with the labels:
LARGE LEGS - Mid-section, labeled MR & ML
MEDIUM LEGS - Front, labeled FR & FL
SMALL LEGS - Rear - labeled RR & RL
As for which side is left or right, orient the AT-TE as if you're the driver. The legs on the right side are from front to back - FR, MR & RR. The legs on the left side being FL, ML, RL.
Additionally, the points where the front & rear legs attach can be moved in order to more easily access the attachment points. Pull those away from the hull slightly to get the attachment points completely unobstructed.