I would recommend lots of homework and tons of looking at what others have done. What worked for them what didn't. How what you want will apply to your area/space. In my mind the first important steps are...
location: where do you want it to be, can you see it, will you have fast easy access to it does it put your fish at risk.
Size and shape: you mentioned 1200, will this be too small when you find 4 different types of koi that you fall in love with. Smaller the pond the harder to keep more fish in it and keep the water quality where it needs to be
Filtration type bog style vs mechanical, or a combination of both. Where will it sit how much space will it need. If it's mechanical can it be made to look nice or hidden to not distract from the pond.
Style: Do you want a natural look with large rocks fitting for a natural environment shaped in a way that looks like it blends into the environment or like you stepped into a jungle oasis. Or a sleek modern look with few plants and rocks and modern lines
Then once you figured these elements study each decision in depth to make it exactly what you want.
Keep in mind some things you can spend less and some require you to not cut corners. Lime your liner this needs to be high quality and more than the size you need. And round the corners if you do cut it.
looking forward to the build best of luck.
