With the Barbell Back Squat, the first thing you're going to need is a station of some sort to hold the barbell. Then, of course, you'll need a barbell. When I set up my platform I like to make sure that I'm setting the barbell right about in the middle of my shoulder before I lift it off the rack. That way when i'm setting up i don't have to tippy-toe myself off the rack with all that weight on my back.
To begin, I'm going to set my barbell right on my upper shoulder and as i stand up i'm going to lock my elbows down and in. Make sure to keep the barbell in place as you take one to two steps away from the platform.
From here your feet are going to be about shoulder width apart. I like to put my toes out a little bit to help track my knees so that my knees aren't going to be caving in.
As i squat down, the first thing that's going to happen is my hips are going to be sent back just slightly and then i'm going to drop down into my squat. From here i'm going to drive from the bottom of my feet push myself up, lock my legs out (but not 100% - maybe about a 95% extension).
Make sure with each rep you're keeping your weight driving into your heels so you're not coming onto your toes. Whenever you're done walk forward and re-rack that barbell.
That's going to be your barbell back squat