As part of our mini bathroom remodel, I made a curtain for underneath the vanity.
I cut up an old curtain panel we were no longer using, hemmed the edges and added some white pom pom trim to the bottom.
To hang the curtain I came up with a super ghetto awesome idea for a curtain rod, using things we already had around the house- a dowel rod and two binder clips. I screwed the binder clips into the cabinet base and the wall, then wedged the dowel rod into the clips. I reasoned it didn't need to look nice since no one will ever see it, unless they decide to crawl underneath the vanity. Of course if you're willing to spend a few bucks you could just purchase a tension rod.