depends how natural you want the edge to look, i used cut turf and rolled it down over the edge below the water level, lookes totally natural.
The one thing Jannie hasnt mentioned the the oxygen/nutrient balance and how to maintain it.
If you dig hole , put a liner in, pump in 5000 litres of tap water it'll look great for about 4 days. Then the algae notice theres this huge food supply and partee in it. The water will go like pea soup. Two things are happening. The algea are sucking up all the nutrients out the tap water, and in the process, using all the oxygen too.
If you do subsequent management right, this will only happen once.
First thing you have to realise is that tap water is BAD BAD BAD!! Its chockers with nutrients, and adding it to a pond is inviting the algea to breed and make it murky. If you need to add water, add rain water, only add tap water as a last resort.
The idea here is that you need to establish plant life that oxygenates the water, but removes excess nutrients. SO you need to throw in a huge mass of oxygenating plants from day one, and battle will commence between the algae and the plants. Eventually, the nutrients disappear out the water, the algae die, the plants triumph. The water clears, and oxygenates. From now on, the plants will suck excess nutrients the moment there is any, and the algae wont get a look in.
You will ALWAYS get an algae bloom in spring, because the plants slow down in nutrient uptake in winter, nutrients build up, and then when the water warms up the algae are quicker off the mark, but in a well balanced pond, it'll only last a week at best, and then the water will clear. Unless you add tap water, then you're adding nutrients and the algae/plant war resumes. You can resort to electronic warfare- theres UV lights that you can pump the water past that kills the algae, but they are fairly expensive and the tubes are designed to last only one year.
Then you have to consider fish. The natural oxygen levels in a small pond can be too low to keep fish going, so you need additional oxygenation, and thats done by breaking water up in air and slopping it about - like a fountain, or waterfall does. Also realise that the smaller the pond, the lower the oxygen reserves in the water, so if your pond pump stops, your on a timer. Even something like a deep area of low pressure passing overhead can make the oxygen fizz out the water and kill the fish (like a pop bottle fizzes when you lower the pressure over the pop by opening it). On that basis i would recommend you have at least half the floor area of a pond at least 4 foot deep. If you want to find out where the water level lies as you dig, you fill a length of hose with water, and it acts as a water level indicator. Just fix one end to a stake where you want the level to be, and when full of water, the levels at each end in the hose will always equalise.
Also, you cannot keep fish and frogs together, the fish view the tadpoles as moving snacks. You need two ponds at least. You will have to add imported frog spawn for 3 years. On year four, the frogs that were born in the pond from the imported spawn from year 1 will come back to your pond and spawn there naturally, and you have a permanent frog colony. Move the frog spawn out the fish pond if any spawn in there.
In the summer, you have to consider the rate of water loss. The more surface area exposed to air, the faster it will evaporate. I have a 5000 Litre pond and without the plant cover it now has (about 80%) it can lose 4 inches of water on a hot summer day. So i let pond weed cover my ponds most of the time, because it provides cover for fish, slows down evaporation, and protects against the blooming heron colony near here. I have a clear area i can see the fish, because i have a fountain and water fall, and pond weed will not grow near moving water.
All this is learned from experience, I have 4 ponds...........