Sit into a regular back squat but at the bottom hold for 3 seconds, squeezing your core & glutes tight as you breath, then stand up and repeat.

Weight Recommendations:

Beginner: Try just the bar first or dumbbells
Intermediate: 10-25lb plates on each side of the barbell
Advanced: 30-55lb plates on each side of the barbell

*Modification: Do this with dumbbells