Step 3: Trim lumber
I wanted to present a narrow edge at the front of the picture, with the wider face positioned to cover the roughly 2" deep stretcher frame. (This scheme should become more obvious when you see my later assembly pictures.)
The edges on the boards were quite rough and the boards themselves were not square, so I needed to be sure to trim along the entire length of the board. To square the boards, I judged by eye which side was straighter, put this side against the table saw's straightedge and made a cut just deep enough to cut at least one blade width over the entire length of the boards. I then flipped the board over, adjusted the straightedge to to cut another blade width narrower and cut this side. By doing this, the two long edges of the board were now parallel and straight.
Make sure to move the wood smoothly through the saw, as pauses may allow the blade to burn the wood.