It slides into the dovetail notch on the rear of the receiver, from the left to right. The notch is tapered. There is a setscrew to the left of the large adjustment screw on the top of the sight. Slide the sight in, center it in the slot and tighten the setscrew.
You can put a drop of blue Loctite under the sight, and another drop on the setscrew before you tighten it, if you want better security from it loosening again.
Unless you happen to get it exactly where it was, you'll need to readjust your windage adjustment a bit.