For internal corners, you can cut a 45 deg mitre on one piece, with either a circ saw or mitre saw, then follow the line of the cut on the surface, cutting away the mitred bit with a scrolling blade in a jigsaw (90 deg). This piece will then butt against the other making the "mitre".
well wot i do is mark the cut in pencil (2h)luv em, then cut the mould (only )at 45 degrees, then undercut the bottom with panel(eer jack) saw then cut line 2 mould with copping saw