I'm thinking of posting a devlog or something similar that outlines the process in one place, but basically if you google search for your Windows version and SDK you should be able to find the download link for the SDK. Then download and install the SDK. This will include the signtool for your version of Windows.
Once that is done, point Godot to the signtool in the editor settings and make sure you have filled the Company/Developer details in your project settings.
When you export to Windows, Godot will use the sign tool to "sign" the application using the details from the project settings.