Hi Staci -
First off, you just need to engage your core enough to stabilize your pelvis. Anything beyond that will introduce unnecessary tension (like in your glutes). A tip that seems to resonate with many of my clients is to imagine you’re zipping up a tight pair of pants. You engage the low abs enough to pull them in; that’s all.
A good exercise to help isolate those muscles is described in the book - the floor exercise. On your back, with knees bent, press your low back to the ground and walk your feet away from your bum, keeping the low back pressed to the ground. Only go as far as you can hold it. Then march up and down in little steps while keeping your low back pressed to the ground. This will strengthen your low abs and take the glutes out of the picture.
Let me know if that helps!
Certified Chi Running and Chi Walking Instructor