I had NO IDEA how to make the cuts work and I searched and searched for a tutorial on how to do it, but the only thing that I came up with was how to do the typical outward facing bullnose. I took what I gathered from that and went with it. On a typical 90 degree corner you would miter your edges at a 45 degree angle each but for this I cut a middle section with 22.5degree angles on each side and then my two side sections at 45 degrees.
When you trim the side pieces like above don't set you saw at any angle, just make a straight cut. I don't think this is the "proper" way to do corners like this, and if some manly man saw this he would probably cringe all over. Cringe, baby cringe!!