The first thing you need to consider is the location of your new garden pond. The ideal location would be in shade or semi-shade for at least half the day, in an area that will not receive excessive runoff ie. the bottom of a slope and is close to power & water supplies. The next decision is shape & size. A kidney shape is very popular or a squashed figure of eight. Try to keep your shape simple and elegant, complex small coves and inlets tend to look a bit rubbish when all is said and done.
Go large! Make it as big as you can, also consider what size pump & filter you will need at this point. Calculate the size of liner you need as follows – length+max depth+2ft & width + maximum depth + 2ft. This will give you plenty of liner to play with. Mark out your shape on the ground with a hose and remove all the soil to a depth of about 8″, do not dig straight down, use a slight angle to help keep the sides stable. Then you are going to create a shelf about 8-12 inches wide for marginal plants to live on by once again digging down, this time to a spade depth . It doesn’t have to be all the way round, its normally best concentrating on the back & sides, the front is best kept clear for viewing. Now you need to take some time to get the top of the pond as level as you possibly can, this is very important. The water will show any discrepancies and it can look very bad!
A good way is to knock wooden pegs in every 18 inches or so around the edge of the pond, using a spirit level. From standing in the hole carefully remove the soil with a spade till all pegs are of equal length. Check the sides and base for any sharp stones, roots etc and remove as needed. A good 2″ layer of builders sand should be used to line the excavation and the liner placed in and the pond filled up. A butyl rubber liner would be the best, but of course is the most expensive. Take extra care with PVC type liners, make sure there is no liner exposed to direct sunlight, or in a few years it will be brittle and will crack. Take some time to fold & fit the liner as best you can as it fills to minimize wrinkles. Now you can place your electrics & filter etc and connect them up.
There are many different ways to edge your pool, paving slabs are great for the viewing area and rockery stones for the back & sides. The liner should be brought up behind them & backfilled with soil and plants. This hides the liner completely, the worst thing is a pond liner on display and often the guarantee does not cover UV damage. That’s it! The next step is stocking your new garden pond with fish & plants.