Making mitered corners on large casework is not a simple task. Especially if you don’t have a sliding table saw or a high quality track saw. Long and wide boards are hard to maneuver or hold pressed against the saw table. They are also always cupped, twisted, etc. (I know, it just keeps happening to me!) which causes curved cut line. This is a jig that I made for such situations:

It is a plywood 45 deg. wedge with guide rail. First, I cut miter using table saw (or by hand) slightly oversized (~1 mm) and clamp the jig to the board like this:

I hold the router slightly turned with one handle closer to me and one extended for better tipping control.

The jig is very rigid and will straighten up the board if the latter is slightly cupped. I use spiral bit for smoother operation. Since a relatively large area of end grain is cut you will be able to remove small amount of material at a time and several passes will be required until router base glides along the guide rail. An added benefit is that your final cut will be exactly along the edge of the jig. You can literally split a thin pencil line. The surface of the cut is perfectly flat, straight and smooth. I usually scribe a line on the left edge of the board where router bit exits the cut (left side) to prevent tearouts. This is how the finished miter looks:

A more perfectest miter can not be made by any craft known to man :-)

And even if you own one of those fancy Euro sliders you can find this jig useful for feeding your cattle:

… or sheltering your dog: