What you are refering to is making the jack miter at the front edge of the tread where the return and tread begin. There are various ways. One carpenter carefully bar clamps all the treads together and attaches a guide for his circular saw. Then he cuts all his jack miters at the same time!
A bold way! Some use a sliding table saw jig but this seems awkward to me. The way I did it was I used a large speed square with a 45 angle. Using my HIGH quality jig saw and a stiff blade, I cut the miter using the speed square as a guide. Once I reached the intersection of the 45 and 90, I made most of the 90 crosscut with my sliding compoung miter saw and finished the cut with the jig saw. I found I needed to finess the 45 jack miter cut sometimes and I used a rasp to do that. I used the already made return with the 45 to check for fit. Once it fit well, I simply bisquited, glued and bar clamped the return to the tread. Time consumming but not hard.