I have an older version of Font Creator by High-Logic.
It's wonderful but a bit tricky to use at first. The nice thing about it is
that it will edit pre-existing fonts. What I would do is download a font and
save a backup. (Most fonts are free and available to the public domain with
certain license restrictions sometimes so it's important to read their site
info or their accompanied "Read Me" file, if any.)
Edit the individual font characters and play with the shape tools. You'll
find that there is quite a bit involved and even if it's overwhelming at
first, the help files are a tremendous help. (Note: I'm not sure if it's
easily available anymore but Macromedia Fontographer was actually the very
best I've worked with but I was a student when I used that. As we all know,
Macromedia was bought by Adobe some time ago so it's possible Adobe may have
that or something like it on their site.)