What i would do is take a drill and drill a hole in each corner (it helps to keep from over shooting the corner). A metal straight edge (or something similar) clamped to the wood and placed so the edge of the base plate rides up against the straight edge (place on the outside of your cut so if you stray from the guide you cut into your hole) it then becomes a simple job of cutting with a jig saw.
Otherwise i would use a coping saw (if you dot have a jig saw) and free hand it (you still want the holes in the corner). If you use a coping saw realize that the blade comes off so it can go through a hole, and the blade turns in the frame so you can turn corners (you have no idea how many times i have seem people amazed at how flexible a coping saw is)