1) Split the legs roughly 30-36 inches depending on your height.
2) Drop the back knee directly down forming a 90 degree angle with the back leg and coming up onto the toe of the back foot.
3) The front leg also bends to about 90 degrees, but the front foot must stay flat on the ground.
4) Keep your torso upright. If you tend to lean forward it is due to tight hip flexors. If this is a problem for you spend some time on stretching the hip flexors and do some glute activation and strengthening exercises (bridges, adductor walks, RDL's).
5) Drop your back knee until it is one inch off of the ground. Some people believe you should not allow the knee to touch the floor. I do not totally believe this as long as you only touch it to the ground and do not bounce it off of the ground or rest on it. By bouncing it off of the ground you can damage your knee structure, specifically your knee cap, from the force your place on it by bouncing (aka. slamming your knee into the ground). Also, if you rest on it you will allow the tension to come off of the muscles and rested on the ground. This is not affectively and fully getting the most out of the exercise. However, if you only touch it to the ground gently, not resting on it you will be able to get a slightly greater range of motion, which will make the exercise slightly more challenging. It is your choice I am fine with either as long as they are both done properly.
6) Do not have both your feet in the same line. You are not being tested for a DUI by the state police. Keep your feet shoulder width apart.
As I said before there are many different variations of the lunge. But, the form is the same for all.
Well that concludes my lessons on form of the 5 common movements. I hope you are beginning to clean up your form and have found these posts useful.
Have a great day!