Southwest doesn't have assigned seating. You should log into the Southwest website 24 hours before your flight and check in. You'll either get an A, B, or C boarding pass. The closer to the 24-hour mark you check in, the better your boarding pass.
The first people allowed on board are the pre-boarding folks.
A boarding passes board next and pick whichever seat they want. There are about 40-45 A boarding passes issued. B boarding passes board once all the A are on board, and get to pick whichever seat they want. After the B boarding passes are on board, then the C boarding passes are allowed on.
So it doesn't matter if it's a DING fare, a non-refundable fare, or a free ticket, or anything else. Everyone follows the same procedure.