Well, you start off by finding a way to install custom themes on your Windows OS. Then, you find good themes to install. Once that's done, since it's Windows, there's a lot you have to tweak about the explorer if you want it to look somewhat minimalistic. There's loads of guides on that. Try searching for "windows ricing guide" on the internet.
As far as my build goes, I also use custom icons, which I installed somewhat manually, but there's easier options out there. The browser is styled with an extension called "Stylish", but since it's Quantum, I also had to install it physically rather than just installing it as a userstyle.
There's a lot more I tweaked, but that should be the basics. All in all, it will take you quite a while.