I am no expert, but I struggle with this too. I find a combination of theses things to be helpful:
- tucking my stomach/pelvis in
- reaching out of my shoulders
- pushing my butt/hips back
- digging into my heal and wiggling my toes on the planted foot (if standing)
- relaxing the rest of my leg and focusing on the glute muscle
This is what I find helps to activate my booty instead of thighs/hamstrings, but again, I am no expert. I hope this helps!