Set-up: Find a leg extension apparatus and adjust the seat and pad as instructed on the machine.
Movement: Extend the leg, being sure not to swing through the hips or back. Return to the start position.
Benefits: Generally speaking, squats and squat variations will generate a larger tension on the quadriceps and do so with a better risk profile. As well, squats and similar exercises will generate more carry-over to real life movements than the leg extension. That said, should you wish to isolate the quadriceps - the leg extension is a good tool for the job.