Ummmm, you don't?? SW doesn't allow seat choice when booking. It's a first come, first served type deal.
When you check in, online, at the 24 hr window, you will then get a boarding group...A,B, or C. Then, within that grouping will be a numerical value...A16-60, B1-60, and C1-60. I say '60' only because I don't remember how high each group goes, but it's something like 60.
Business select and full fare passengers get A1-15. When they call you up to line up to board, the A group gets up, and goes to their position. There will be poles with numerical values...1-5, 6-10 and so on, in increments of 5. You find you slot and wait. Then, you board. Then, the B group lines up, followed by the C group. There are many times that those traveling with kids under 4 (and in the B or C group) are allowed to board between the A and B groups.
As you board the plane, you are free to choose any unoccupied seat.
Hope that helps.
Those who choose to pay the addtl $10 for EBCI are checked in, by SW, at the 36 hr window.