There are several different procedures for sequencing a segment of DNA.

Dideoxy sequencing  relies on copying a single fragment of DNA many times but each copy is prematurely terminated.  This produces a series of nested DNA copies, each differing by a single base in length. These fragments are separated on a polyacrylamide gel and the DNA sequence is determined by reading the the terminating base of each fragment on the gel.

This process is presented in diagrams and photographs.   Slides 2-14 diagram the process of creating nested fragments.  Slides 15 - 25 are photographs of the preparation  of a polyacrylamide gel for separation of these fragments and results obtained after electrophoresis.