Bridges
Lie on your back with your knees bent and feet flat. Press your hips up.. The goal is to get your body straight from knees to shoulders. Hold this position for 3-5 seconds, then slowly lower back down.
When this is easy you can progress to alternately lifting one leg at a time and holding it nice and straight The focus then is to not allow your hips to drop as you alternately lift one leg at a time. You'll bridge up using two legs, then stabilize and lift one leg and then the other. When you're ready to lower your hips you'll do that with both feet on the floor.
When this gets easy you can progress to single leg bridges but you'll need to really focus on a nice tight core when you do this. Repeat as directed by your coach.