shape up!; act properly!; cut it out!
A stern, often exasperated command telling someone to stop their inappropriate behavior and behave sensibly. Used by parents, teachers, or superiors.
Stop being late already and shape up!
Stop crying all the time and pull yourself together.
いい加減にしろ is a blunt, angry command to stop, while いい加減にしなさい uses the softer なさい form typical of a parent scolding a child.
From いい加減 (irresponsible, half-hearted) + に + しなさい (imperative of する). Literally 'do it in a proper manner', but used idiomatically as a scolding phrase.