This is for all you DIY enthusiasts out there.
Back in the days when I was working as a Landscaper I occasionally dug garden ponds for people. Of course, if you dig a hole in the ground and try to fill it with water, the water quickly soaks away. The solution is to line the hole with some kind of impermeable liner; the best ones are made of Butyl rubber. The size of the liner needed is simple to calculate- max width plus twice the max depth, max length plus twice the depth, with a bit added all around for overlap, but for those who have troubles with math, Google provides a handy tool.
It's a good idea to plan the pond with two levels, a deep end and a shallow end, just like a swimming pool, except the bottom of each level should be flat, separated by a shelf. This is so it is possible to add water plants in perforated containers and they will stay put.
Ok- hole dug, liner installed, you just turn on the hose and Voila! Right? Wrong. It used to pain me to see about 6 inches of liner showing all around the edge of a new garden pond, with slabs or flat stones trying to imitate a natural border. The solution is simple but will take a bit of work, but hey, you need the exercise anyway, right? We'll assume you dug the hole with sloping sides, otherwise the frogs won't be able to climb out and the poor things will drown. Carefully spade down vertically all around the perimeter and create a level horizontal shelf about eight to ten inches below the level of your lawn- it should be about eight to ten inches deep. (if you did not site the pond on more or less level ground you have a lot more work to do. All around the perimeter of the pond carefully use your spade to remove a further section of turf, in squares about twelve x twelve x two inches thick. Set these well back out of the way. Take a break from digging and collect a great many flat stones of various sizes.
Go buy the liner- most large garden shops stock them or you can buy online. Lay it out over the pond and start filling; it's a good idea to anchor the liner with a few stones so that the water will stretch the liner as it fills. Fill just up to the shelf you have created. Now, get down on your knees and build a rough dry stone wall with the stones you have collected on the shelf on top of the liner- save the biggest ones to go on top and tuck any old rubbish stones in from the back and fill so the top ones remain level. Build this up to the level of your lawn. The remaining liner you now lay flat and cover with the two inch thick pieces of turf you removed. Any remaining liner sticking up, cut until it is lying completely flat under the turf.
Finish filling the pond and stand back and admire. In a few months it will look as though it has always been there and any run off due to rain etc. will go under the grass.