The client wanted a seamless floral pattern based on some of the nostalgic arts-and-craft-style wallpapers. After some research I found a pretty good William Morris pattern online to use as reference.
I started by drawing the leaves in Adobe Illustrator. The outer contour is a stroked line and the veins are filled shapes. (The left shows the drawing and the right shows the vector lines.)
The two parts are assembled onto a dark background. Note that the stroke vector is duplicated behind the composite image to build depth.