Installation of a pre-formed pond is fairly straight forward but does require some patience. It is important that the pond be well supported on all sides so you don't want to over dig if you can avoid it. Leveling is also very important. Even a 1-inch difference between one side and the other will be very obvious once the pond is filled with water. If there are any gaps around the pond after you fit it in place use sand to backfill them, not dirt. Dirt will settle over time and lead to stress on the sides of the pond.
Mark the rough outline of the pond. You can use a garden hose, spray paint, wooden stakes or anything else that will show you roughly where to dig.
Start digging 6 inches inside the marked area to start. Remove all of the loose dirt as you go. Be mindful of the depth. You don't want to over dig.
Once the bulk of the dirt is removed it's time to start the edging work. It's best to double check that the shape and size of the marked area is accurate before you start this step. Make straight shovel cuts in the dirt as you go. Continue to remove all of the loose dirt.
It's almost a given that the shape of the hole will not be perfect from the start. Place the pond in the hole to check where you need to remove extra dirt so it fits as tightly as possible.
Once the pond is in place, use a level to check the height of the edges all the way around. Even a small height difference will be very noticeable once the pond is full of water.
DO NOT BACKFILL WITH DIRT!!! You will almost always end up with areas that aren't as tight as you would like. Use sand to backfill the gaps. Dirt will settle over time leaving gaps which will put stress on the pond.
You can lay flat rock around the edge of the pond to hide the plastic. This is not really necessary but it will make the finished pond look more natural.