You would walk on the tarmac up to the plane door. They would bring big stairs. Although some planes did have their own retractable stairs, namely the 737 and models with a back stair, such as the 727 and maybe the BAC111 and Caravelle.

Of course going back earlier, my first flight was on a DC-4; second one a DC-6, must have been 1961 or 1962. But at that time these had already been withdrawn from premier service.