@Kalmaro In a nutshell… The animators drew the action on paper with pencil, frame by frame. The drawings were then sent to be inked and painted on a separate celluloid (transparent plastic) sheet by tracing. Finally, the ‘cels’ were sent to the camera, where they were placed on top of a background painting and one picture was taken. Repeat the process thousands of times and you have a cartoon.
(Back in the day, Disney animated films had around 150,000 separate drawings/paintings)