A reasonable readiness check is that you can control your bodyweight in a squat and a single-leg squat, land quietly and stably from a small hop, and have no current pain in the loaded joints. Start with low-intensity hops and build volume before intensity.
Landing forces are several times bodyweight, and the ability to absorb them is what protects you. Noisy landings, knees collapsing inward, or a wobbly single-leg position all indicate you should build control before adding intensity. This is covered in Landing Mechanics and Force Absorption.
Start with two-foot pogo hops and ankle hops, keeping contacts small and quiet. Add countermovement jumps and box jumps, where you step down rather than jump down. Only once those are comfortable should you progress to single-leg hops, lateral bounds, and eventually depth jumps.
Twenty to forty ground contacts per session is plenty at the start, twice a week, placed at the beginning of a session when you are fresh. Increase contacts before you increase intensity, and never both in the same week.
People with a recent lower-limb injury, significant knee or Achilles history, very low current training volume, or who are carrying substantially higher bodyweight should build strength and landing control for longer first, ideally with professional guidance. The progression is set out in Introduction to Plyometrics.