Place it sticky side down over your arrangement. Do not worry about lining up edges exactly. You will probably get some wrinkles. Just press everything down as flat as possible and your place mat will look beautiful. Trim around all edges with scissors.
TIP Have someone help with the contact paper.
You can each hold 2 corners and it will be easier to apply. IDEA Make decorative edges by cutting scallops or applying colored tape. IDEA Cut into several different shapes pieces and make a mobile to hang in front of a window.