Get help from a qualified salesperson or follow the instructions closely - with a little practice, you'll soon be able to fit the seat quickly.
Put your weight on the seat as you fit it, and remove as much slack as possible from the belts.
ISOFIX seats that can clip straight into a car can be quicker to fit than seatbelt designs, but they often need a top tether or leg to be properly secure. Many seats also have clear indicators to show when everything's connected.
Never put a rear-facing seat in front of an active airbag, and check connections, straps and belts are secure before every journey.