Squats are great for sculpting a rounder bum. For this exercise, you might want to try using slightly heavier weights. To find the right weight, simply do 10 squats. No more, no less. Through trial and error, you will figure out the ideal weight.
Performing the exercise: place the bar on your shoulders and draw your shoulder blades together by contracting your trapezius muscles so that the bar is resting on your muscles and not on your bones. Start standing upright, facing forwards, and with your feet parallel and hip-width apart. Bend your knees and hips and push your bum backwards. Your chest will naturally lean forwards slightly to keep you balanced.