Secure one piece of moulding square against the corner you are trimming with #6 or #8 finishing nails or a staple gun. Once you have one parallel installed, it is time to fit the joining piece.
To do this, fit your Dremel Rotary with an EZ Drum and Sanding Band. The EZ Drum makes it easy to replace worn bands throughout your project if needed. With your tool set to high speed, sand the excess material exposed by your cut piece of trim.
The #408 Sanding Band is low-grit and will allow for more aggressive material removal. Remove a bit of material and check for fit as you remove incrementally more material at a time.
Remove material carefully as you near the end of your coping project. Replace the #408 Sanding Band with a less aggressive, higher grit Sanding Band like the #445 for fine-tuning your fit.