If your going to just have gold fish do DIY methods as best you can but if your going to have fresh water rays or expensive koi I'd recommend research further or doing it professionally. Structure consists of 45mm rubber lining(recommended carpet patting underneath). The patting is to help you avoid piercing from rocks you use to line the pond. Filtration is key not only for the success of your aquatic life but also for water clarity so you can enjoy your pond rather than just maintaining it. For the filtration I recommend a bio filter/waterfall because the water play helps oxygenate you pond. It is also lessens the amount of equipment you have to hide. A skimmer is key for maintenance and for your fish. Some leaves that fall into the pond may raise the pH levels too high for your fish and the skimmer can also be used for extra filtration by adding bio pads inside the components. That's the simplest I can break it down. Depending on the purpose of the pond the rocks you use make a difference the plants you use mater and the pond ground cover matter. For the bottom of the pond I recommend small pea pebbles to help the debree settle.
