Need some help with how to lunge with a barbell?

Muscles Used
Hamstrings, Glutes, Quadriceps.

Technique
Load and hold a barbell as you would for a traditional back squat, with the bar resting on the flesh of your trapezius rather than up on your neck.

Keep your chest tall throughout and shoulder-blades pinched down and back. With one leg take a long stride in front keeping feet in line with the hips and getting your front knee at a right angle. The rear knee should end up just above the floor to maintain tension within the active muscles.

From this position, power back and up off the front leg to return to a standing position.

Notes:
You will need to keep your core tight and braced outwards throughout to maintain balance.