Time for the bottom of the shorts. You should be looking at the shorts inside out.



Now I did the crotch like a triangle and that wasn't good. I do not know for sure if an arch is much better, but it should be at least a little better :)



Find the halfway point like you did with the button holes. Also decide how far down you want the crotch to start. You can see I do not have much space below the crotch and that is why I think it should be longer than I did it.



Once you have your crotch starting point, mark it. You are going to go off to the side a bit on each side like the button holes. You can see the drawings for what I think you should do. Don't sew the crotch arch too wide because you are cutting into your leg room then. That is why you might want to go wider than I said.



Sew the arch, then cut up the center without cutting into your sewing. I didn't have a lot of room so I used fray check on it. If you can serge it, do that.



Once the crotch is done, fold up around the legs and sew the seams. Nothing special, just fold them up and sew.