Liner is the easiest way to build a pond. The folds can be hidden with rocks. The filter is very important. Over sizing is best. The one of the easiest filter to maintain is made by Savio. They use the plastic straps as media. With a drain valve added to the filter, it will make flushing the filter very easy. It will take only minutes and will never jam up. It is pricey but time is money. Or you can make your own filter if you don't have a budget for it. A pond should be about 36" deep for koi but deeper the better. Out here, raccoon is a major problem and that is the only way they won't go in. The sides should be fairly sheer. Raccoons like to wade not swim. There are other predators such as birds. A hiding cave or structure will help shelter the fishes. The bottom of the pond should have a lower side to it. It allows water and debris to migrate to the lower end. It makes maintaining it easier. If you are dealing with rocky soil, you may need a smaller jack hammer or demolition hammer to dig. You will get out twice as much soil when you are digging. The soil will loosen up and you will get about double the amount of soil out. At least a small waterfall should be built to help with aerating the water otherwise you may need an air pump and air diffuser as part of your filter system. Without it, you may not get enough oxygen in the pond. There are many ways you can build a pond and type of equipment to use. Apply common sense into it and it will make things easier for you in the long such as having the sides of the pond higher so rain don't wash dirt into your pond. Do your research and have fun doing it. Good Luck.