Clear plastic sheets can be cut using common scissors or a box cutter knife. It is advisable to place something under the sheet to be cut to avoid cutting or marking other surfaces. Using a cutting mat and a rotary cutter, normally used for quilting, is a great way to cut plastic sheets.
If a precise measurement is required, then a permanent marker and tape measure or carpenter's square is required. Complicated shapes can be cut using scissors or a rotary cutter. Box cutters do not offer good maneuverability and are mostly just used for cutting straight lines. Plastic sheet cutting tools that will cut even thicker sheets are available at Home Depot.
It is important to wear gloves when cutting plastic sheeting with an open-bladed tool. Always keep hands well away from any cutting tool. Sheeting can be folded many times so that minimal cuts are required.
Some plastic sheeting may come with a cheap cutting device for just cutting straight lines. Plexiglass is the most common type of rigid yet cheap plastic sheeting and requires a tool that is capable of scoring the surface. Alternatively, a circular saw with a fine blade can cut through most Plexiglass sheeting.