Works even if the legs are NOT straight cut. Just open the seam a bit and taper to the hem line. If you can do the first part, you can most definitely do the tapering. It will be very clear immediately just how much you should taper.
As for doing a flat felled seam, easy peasy. There's only three steps to a flat felled seam. You can even Google video's as to how to do that. Once you see it, voila. Just like learning to make a quilt block. See it, do it.