Are all the corners going to be a 90 degree angles?
If it's a basic 1 1/2" moulding with 90 degree angles for the fireplace then just cut them at 45 degree angles, if you're talking about a basic fireplace then it's extremely easy to do but need more info to give a better reply.
PM me or post more here on the site.
Insanity is a long and winding road ... I think I finally made it there.
Larry,
You got a good power compound mitre saw with a good sharp carbide blade?
I've got a few, I could loan you one!
also, get some cheap scraps to practice on
If you have any inside corners, get a coping saw and practice with that also