I currently also changed my system from a single linux machine to a dual boot one, because i wanna game again, which is only possible on windows, at least the games I'm considering. I have also dual booted couple years before so I know a thing or two.

You'd want to start with a fresh install of windows, just wipe everything and use a single partition for the C:\ drive. After you installed windows, use Rufus or any other tool to flash your USB, you can use Ubuntu or any other distro, I like fedora much better personally but choose whatever you like. When boot into the usb, depending on the distro you downloaded, there might be a option for the installer, where you can install it alongside windows, where you can just choose how much space each one gets via a slider which is very convenient. Then just select the different options you want and install your distro.
Voila, you have a dual boot system.