There are two good ways to create and cut an oval.
The first one was an idea from Jin7, our Japanese CRI in this community, where he creates his ovals by computer.
Choose your drawing program (e.g. MS Paint) and draw an oval up to the desired dimensions. Then print it out! If you need a bigger oval, you can draw half an ellipse and print it twice. Cut the oval (or both half ovals) out from the print, place it on the contact paper (back side) and draw its contour on it. Then cut it out with cutting knife or scissors.
The slower you do this, the more accurate.
My way to do this is using a compass, a ruler and a pencil.
Search the internet for a site where you can find 'How to create an oval/ellipse'. Follow the instructions and draw this on the contact paper (back side) with the desired dimensions. I always use a cutter to make a small slice in the paper, just big enough to put my scissors in and then start cutting.
In both cases, keep the inner oval that you had cut out to use for future projects.