Butyl liner is flexible and can be made to the shape of your pond. It is quite heavy and best laid out on a warm day to make it flexible and malleable . Start from the centre and lay the liner outwards. The aim is to get it as flat as possible and run it up to the top of the pond and just over onto the ground, bearing in mind the need to hide the liner to ensure the finished result looks professional.
Preformed liners are made of plastic or glass fibre and are rigid, long lasting and need to be fitted exactly into the hole and level.
For all ponds level is crucial: it is worth spending time levelling the pond because if you fill up the pond at the end stage, and it is not level, it will show. Water will find the level and it will look odd. A spirit level needs to be used to check the pond at the bottom and at all points to ensure it is 100% level before fitting the liner.
A butyl liner needs to be laid from the centre of the pond outwards pleating carefully on shelves to get the liner as flat as possible.
Retailer web sites explain how to calculate the size of pond liner needed for your intended pond. Liner is expensive as are any mistakes if there are miscalculations. Good retailers will assist in the calculation if you call them with the pond dimensions.
Fill the pond before trimming off the liner. Use bricks along the rim to hold it in pace and then fill, settle and then trim off surplus leaving an overhang which will need to be disguised with plants and stones.
You are then ready to fill up the pond with water. Initially for a short while it will be clear and then turn green. It will take time for the natural balance of the eco system in the pond to return the water to clear. Although in the first instance the only practical way to fill the pond is to use tap water if at all possible after that fill and top from a water but using natural rainwater.