You need to determine how much polythene sheeting you will require for your pond, including the length, width and depth. Once you have these measurements, add a foot on both in length and width to ensure that it will not be too short, you can then purchase your polythene sheeting
Place a pond underlayment into the pond area. This will protect your liner from rocks and sticks. A cheaper alternative is newspaper however this could potentially shorten the lifespan of your liner
Lay your liner carefully across the pond from the middle, ensure that all the shelves are covered; this should be done as carefully as possible avoiding any folds and wrinkles
Fill the pond with water, whilst filling, check for wrinkles and folds, most creases should eventually straighten out, when filling the pond take several pauses providing settle time as the waters weight increases
Place any stones and coping stones around the shelves of the pond and liner to blend and support the liner
So there you go, a step-by-step guide on lining your new pond.