Go to a fabric store, or the fabric section of Walmart and buy one of the cutters that they use for cutting material. It looks a lot like a pizza cutter (a wheel does the cutting), and works amazingly well on our covering materials.

Yes use covering material, not paint, and yes, use a metal straight edge to make straight cuts.

To make both side exact, cut two pieces of covering material large enough for your design, then lay them down on the glass (or cutting mat) back to back and cut them both at once. That way, once they're cut you'll have two pieces, mirror images, that are exactly the same size and shape.

Spray the surface of your covering with Windex, or detergent and water solution (where you want to lay the trim), then position the trim piece. The Windex will allow you to move the trim around until it's right. Then squeegee out any excess from underneath, wait 24 hours, and iron it down.
Dennis-