Southern transfer or blotting. In order to have a permanent copy of the fragments, a sheet of nylon is placed on top of the gel. Paper absorbs the liquid in the gel so the fragments attach themselves to the nylon sheet. At the same time, the DNA is treated with chemicals or heat to cause the fragments to unzip into two single strands. Interpreting the DNA print.
Most exclusions are obvious.
If not so obvious, the bands are measured through the computer. Determining a match. Labs conduct population studies based on samples from blood banks. That provides a database for determining the odds of a match within the general population. Some argue that there are variations within racial and ethnic groups that skew the odds of a coincidental match - a position Simpson's defense is adopting.