Before starting installing the lined pond, what you need to do is to dig a thin layer of topsoil or turf of about 45cm (18in) wide with spade on the outer side of the pond site. Removing the strip will form the edge of the pond; the flexible liner will overlap this strip along with pond edging, bricks, paving slabs etc.
Now you need to excavate the entire hole for the pond. Lined pond mostly does not have any particular degree of slope, i.e. to any angle from 70° to 0° for sloping pond or home made beach. Make sure that you shape the shelves as you dig down the hole. Allow minimum width of 30cm (12in) to entertain the standing planted aquatic baskets.
In case the soil is very soft and sandy, it is suggested to build a supporting back wall to the top shelf. This wall will keep the edge of the pond safe from any damage later. What you need to do is to lining the back of the wall with bricks or building blocks and secures them in place with mortar or cement.
It is important to level the shelves while positioning the pond. As you proceed with digging, keep checking the level of the shelves to be horizontal with spirit level. Level is important only when you want to place the planted containers there, otherwise it is not necessary to level the bottom of the pond.
It is better to provide some underlay like a layer of sand builder or soil about 5cm (2in) thick on the bottom before laying the rubber liner or the butyl. You have to be sure that the hole is deep enough to accommodate this layer of sand builder. There are other pre made underlay available like proprietary underlay felt or strips of old carpet, which can also be used but they are not so thick, so you need to not to adjust the hole to facilitate them.
Before position the underlay take any sharp or large stones away from the hole. Damp the sand so that it perfectly holds its place in the hole. Similarly, if you are using carpet or felt, lay it properly and firmly on its place. Don't forget to run these underlay on the sharp corners like those caused by strengthen back wall of bricks or building blocks.
It is not difficult to install the liner. You can easily install it with perfection by keeping few things in mind. One of the easiest ways is to mold the liner in the dig. Make sure that you smooth out all the creases as your way through. Now, take a hose and pour water in the shallow part and smooth the folds as it fills.
Fill the pond until it is full. Now trim the edges of the liner in such a way that leave enough width all round to provide a good overlap with the edging you need to cover the pond with.