A simple stitch will join two panels but it doesn’t make the finest looking seam. One solution is a second seam adjacent to the first.
The common practice produces a variation of a felled seam. That thick, wide seam down the legs of your jeans is a true flat-felled seam. But a semi-flat-felled seam dispenses with one of the folding processes. Basically fold over the sewing allowances to one side and topstitch through all three layers.
This process increases the seam’s
strength somewhat and adds a pinch of class, but most importantly it makes your seams lay in whatever direction you want. That’s important on something like a seat edge where a seam that flip-flops back and forth would make the edge look sloppy.