I have noticed that back issues of Fine Homebuilding have tips on stick-framed backing for 45° corners. While they’re good methods, they require 4x4s and tablesaws, neither of which I usually have on site.
To make corner backing out of 2x stock, I simply set my circular saw on 45° and remove one edge of a 2×4 stud. Then I nail the ripping onto the stud, filling the corner void (see drawing). This method will also work with 2×6 studs if you make the rip 3/4 in. from the corner instead of right on it. This method wastes no wood, requires no table saw and results in plenty of nailing surface.
Tom Mahony, Kona, HI