Open it like a book, thread the belt, click it shut, and your install is basically foolproof.
The Marathon's ClickTight system is the closest thing to install-proof we've tested: you swing open the seat, route the vehicle belt straight through, and close it to lock in a tight fit every time. Paired with SafeCell impact protection and a steel frame, it's a standout for safety-focused parents who worry about doing install wrong. It rear-faces only to 40 lb, though, so very tall or heavy toddlers may outgrow rear-facing sooner than with the Chicco or Graco. Skip it if extended rear-facing capacity is your top priority, and note it's a heavy, tall seat that won't suit tiny back seats.