How do you install saddle bags on a Yamaha virago 750?

It's going to depend a bit on the specific bag, but a common one drapes across the frame under the seat. So, you would take the seat off, put the saddlebags across the frame, then put the seat back on. One thing to consider: you may also want saddlebag supports. Those are bits of chrome that sit between the inside of the bag and the back wheel...keeps the bag from accidentally rubbing against the back wheel.