One way that I do this is to just sit and sketch. When I get something I like I put it into a scale drawing and then make changes as I hammer. It helps to study some other scrollwork for inspiration. Also for simplicity, one scroll jig can produce different sizes of scrolls and those can be hand tweaked to make an infinite variety. So don't feel bogged with tooling.
Here is a link to some nice books on the subject. I get lots of ideas and instruction from them.
Craft publications