Although I am not left-handed, I know we have a few left-handed calligraphers in our guild and I have been in classes where instructors have said, "please let me know if you are left-handed." In the last few days, I came across a few links that may be of help.
Younghae Chung is left-handed calligrapher, based in Irvine, California. Some may be more familiar with her instagram handle @logos_calligraphy. She has link under the Resources tab on her web site where she shows how she holds her pen and positions her paper. You can also listen to a recent interview with Becca Courtice on Happy Ever Crafter called "What is Copperplate?" She gives a lesson on pointed pen and demonstrates formal, flourished, and what she calls modern Copperplate, while still respecting the basic letterforms of this script.
Another resource is a YouTube video presented by master penman John DeCollibus. He demonstrates a few different ways left-handed calligraphers hold their pen and how he holds his pen as a left-handed scribe.
And finally, there is quite a bit of information on the IAMPETH website on this page.