If you have a rusted out or dented panel on your car, today's cars' exterior panels are an integral part of the body's structure (called "uni-body" construction), and need to be spot-welded into place. Your father's car was simpler, and panels could quickly be attached with ¼ inch bolts.
Many panels are fabricated from high-strength steels that need to be welded with a heli-arc or MIG welder, and that means going to a professional welder at a body shop. Home-welding with an acetylene torch and a piece of coat hanger wire is not an option.
A less-than-adequate job of welding in a new panel has safety impact on all the car's occupants. Newer cars rely on controlling a crash's force by progressively crushing during an accident to absorb the impact gradually. This prevents the occupants from being injured by instant deceleration in a crash. An inadequately attached fender that pops off will not absorb any of the forces of impact.
You should also be cautious when considering using inexpensive after-market replacement panels instead of the considerably more expensive factory replacement panels from the dealer. The after-market panels are less likely to fit properly and require more installation labour, and may use different alloys or metal thickness than the original to provide a different level of protection.