What is the medley order, and what's allowed on the freestyle leg?

Each stroke is swum for one quarter of the race distance, in this order (World Aquatics rules, Part Two, Article 9):

Freestyle leg of a medley: the swimmer may swim any style other than backstroke, breaststroke or butterfly, and the body must be on the breast except while turning. The swimmer may push off the wall on their back, but no kick is allowed until they have rotated past vertical onto the breast; after that, kicking (including butterfly kicks) is allowed.

Each section must be finished according to the rules for that stroke. For example, the butterfly-to-backstroke change in IM needs a two-hand simultaneous touch, and the backstroke leg must finish on the back.