www.makemkv.com

Just playing around, I successfully created a backup up a Blu-Ray Disc.
Now I'd like to make a MKV using the just created backup (instead of
using the physical disc in the optical drive). How do I do that?

File->Open Files, and browse to where you stored the backup. I do not usually use the backup feature, so I do not remember which folder you browse to, however.