Drawing wavy border lines?

[rich2005]

First a caveat: For a biz card or a flyer, that is eventually going to a printer, a graphic created in a vector application such as Inkscape is preferable.

However, this is a Gimp forum, so a Gimp reply.  Use the paths tool to create the curve, then path to selection and fill the selection.

Three and a half minute example: https://youtu.be/bxwIIQeDgqI