Basic lunge: Start by keeping your back straight, with your shoulders back and relaxed and chin up. Be sure to engage your core. Step one leg forward, dropping your hips until both your knees are bent at about a 90 degree angle. Your front knee should be directly over your ankle. You don't want this knee extended over your ankle. Keep your back knee hovering over the floor, but not touching it. Keep the weight in your heels as you push back up to the starting position.
Now that you have the basics down. There are several modifications to increase the challenge.
Backwards lunge: Our bodies are so used to the forward movement from day to day use. Change this up and instead of stepping forwards, lunge backwards. This will work different parts of your muscles and help improve balance.
Lunge with bicep curl: Grab a pair of hand-weights and do a bicep curl as you lunge forward. Keep your elbows close to your body. This will work your upper body as your work your legs.
Side lunge: Step to either side and lunge to change up how you normally work your lower body. Instead of stepping forward, step out to the side. These will work your glutes, hips and thighs differently.
Pulse lunge: Do a typical lunge. Before you come back up, hold the lunge for about 10 seconds and do small pulses here. You will feel the burn.