The barbell squat is great exercise for your leg day. It is targeting your quadriceps mostly, but it is not only for that. The barbel squat will also bring better mobility and stability for your full body, and for that is one of the most important lifting exercises you can do.
To begin with the exercise, you will need a barbell placed on the top of your traps (shoulders), with the proper weight from the sides. Make sure you do not overweight the barbell, because you can hurt parts of your body. Once you have the barbell on your traps, make sure your chest is up and your feet on a place that will keep you control the balance. With your head facing forward like on picture 1, you are ready to start the exercise.
Slowly start lowering the barbell, by flexing the knees. The most important thing here is to keep your torso upright as much as possible. Descend all the way down, until your upper legs contact your lower legs, like on picture 2. Once you are there, make a short pause and you can slowly start pushing your weight up. Make sure you have your legs straight before you do another rep.
Make this exercises in 4 series. In each of the series, make about 7-10 reps.