Wet the leather before you lay the vellum on it to trace the design onto the leather. By wetting it the leather becomes pliable or soft which will accept the tracing marks.  After you have traced the marks onto the leather go over the marks with a stylus to make the imprint deeper. [2,3] Note: Before you start the cutting process make sure that the design is exactly the way you want it because once the leather is cut there is no going back.
Now that you have traced the lines of your design with the stylus, you may begin cutting the leather using a tooling slant edged swivel knife. Please make sure this knife is fairly sharp or you could very easily injure yourself during this process. Hold your knife perpendicular to the leather so you have an even cut mound on both sides of the cut. Follow your lines. 
Finish your cutting process  and I will be back to show you the tooling process! Good luck!