Starting A Small Pond. How Do I Start?

[george4593]

I have a 150L pond and 3 goldfish which are outgrowing their tank so i want to put them in a pond i have 1 shubunkin and 2 comets all around 6" long including tail. but how do i start? what do i need to add to the water for it to be safe for them, and also what plants could i have or what to put in the pond for them to hide?
Help will be much appreciated