Find the spot that you want cinched (might have to have someone help you mark it), measuring up from the bottom so it's even on both sides of the marked points. Say your points are 10" apart; pin a 7" to 8" piece of elastic on the inside at each point, then stretch as you sew it the elastic to the inside of the jacket ~ it might take a little trial and error to get it fitted as you like.

This will leave the elastic visible inside the jacket; if you don't want that, you'll have to make a narrow casing of fabric to cover the elastic.

I made and wore scrub dresses when working. Essentially they are the same as the tops only longer. I liked waist ties on them as they grew and shrunk with me + the task. Just as pixistix says note where you want the waist to be then sew ties into the side seams - I like about 2" wide ties as skinny little ones tend to bend and twist. I agree with you that the looseness of the tops can look a little sloppy and unflattering.