I just want to add to the Follow-up 'kb' made. Perhaps this can help you.
A short while ago on either this forum or another forum, someone responed to a similiar post with a WEB site that explains how to cope moulding. I, fortunately, saved it for me and to pass on to whoever may need it. (I thank the Follow-upper who gave the URL. :) )
Anyway, go to Marley Mouldings. Then, page down to the URL labeled, 'Installation Tips'. You'll be given a list of additional URLs that go through a step-by-step process of installing trim and moulding. SAVE THIS SITE for future reference as a Bookmark from your browser.
I also thank Marley Mouldings for posting this kind of info on the WEB for us to use. (No, I am in no way affiliated with these folks.) It's just nice to see they value good business and want to help current or future customers. Good luck!