If you look within the folders at ...\AppData\LocalLow\Tactical Adventures\Solasta\UserContent
you will see the folders \Campaigns and \Locations. Your campaign is of course within Campaigns.
There, you will see a .json file with the name you will recognize.
There is basically one major step : the .json file must be made into a .rar file (compressed using a free software like WinRAR).
On the Nexus site, fill in the upload form slowly, taking the time to read the instructions there. The form will have, at some point, a button to upload you .rar file. I will not describe all the form fields for the Nexus upload system.
By the way, prepare at least one jpg image used to make your mod page more attractive (you can use the same image used for Steam. Speaking of, for Steam, you simply use the exact same name as the campaign file for a jpg file in the same folder).