I am new to teaching circuit classes and need ideas on how to make it flow. 45 min class, max. 15 people. Thx!

I am good for exercise ideas but need timing help! How long should each exercise be? 45 sec? 1 min? How can I make 15 people flow smoothly? I am thinking 1 min each...2 strength, 1 core then 2 mins of cardio. I need ideas! Thanks so much!