PSD is PhotoShop Document, I’m sure there’s other programs that can open it that’s just what I made it in.
How to make pixel art and how to make basic UI are totally different in terms of skill, UI is a lot easier than most pixel art. Just start by taking their stuff as a base and move on later to trying stuff on your own, generally try and have an idea of proportions before you start as it will make things easier, I find it’s a lot of zooming in and out for tweaking one pixel border width differences and the like for the best look. Remember that more specific questions get more specific answers as well, a sketch of a UI and a colour palette will get you a lot further than saying weaver ui but green for example. In particular say which parts of the ui you want to target, where are these buttons you speak of? Also I forgot to mention but all ui should be based on the 1920x1080 screen size as stonehearth scales the ui based on that screen size to whatever actual size they’re playing at.
I’m pretty sure @voxel_pirate has created at least set of tutorials from modelling to animated. Maybe even more. This is probably a good place to start