The best way to do this is to use a rope or water hose and lay out the shape on the ground. A pond for goldfish or water lilies need be only about 2 feet deep for zones 5 or greater. Ponds built in colder areas may need more depth to keep the pond from freezing solid. Ponds built for koi should be close to three feet or deeper to allow these larger fish enough space. The biggest mistake that most people make is building the pond or water garden too small. A larger pond is more stable and easier to maintain. Keep in mind that a finished pond or water garden will be about 30% smaller than you visualize it. After you have laid out the shape, measure the maximum length and width. Add the depth twice to these measurements plus a foot or two for overlap and this will give you the pond liner size.
If keeping plants dig a shelf around the perimeter of the pond about one foot deep and one or more feet wide. Dig the remainder of the pond with a slight slope to the end opposite the waterfall if one is included in the design.
Pondskimmers should be buried to the proper level beside the pond. A ditch should be dug for the plumbing from the pond to the waterfall or external pond filter. If a pond skimmer is being used, dig a ditch to the external pond pump and from the pump to the external pond filter or waterfall. If you are using a submersible pump in the pond skimmer then the ditch will be from the skimmer to the external pond filter or waterfall.
This can be cut with scissors or a utility knife. You may want to tape any small pieces together to keep them from moving when the pond liner is placed.