Here’s a how-to video and photos on trimming panels with quarter round molding to get really tight miters.   Be sure to always have a sharp pencil and a wet rag at the ready to wipe up any excess wood glue.  It also helps to use a sacrificial fence on your miter saw to help in making nice tight miters.  See below for video link on making the fence.

The first step to trim the panels is to cut an inside miter:
Next, push the long point of the miter you just cut into the inside of the panel. Mark a line of the other side of the molding where it meets the panel, then roll the molding and extend that line.

When the molding is up against the saw fence, you’ll see the extended line and this is the line you’ll be cutting at Hold the molding at the fence and line up the marked line with the saw blade kerf. Try to cut the pencil mark in half.

Attach the top and bottom of the molding with wood glue and a few 1″ pin nails.  Follow the same steps for the sides of the trim.

How to Make A Sacrificial Fence: