There's two ways to upload a modlist, depending on whether you want to use all of your enabled mods, or just some.
If you're managing a supported game, you should see a new button added to the top of your Mods screen: Upload to modwat.ch. Just click that button to immediately start building a report using all of your currently enabled mods. You will be prompted for an optional title, which will be included in the “tag” of your uploaded modlist.
Once the list has been built, you'll be prompted to enter your username and password for
modwat.ch. If you have an existing account, make sure you enter the correct password, or enter a new username/password to create an account. Once you've done that, your modlist will be automatically uploaded to modwat.ch and you will see a notification where you can view your new list.
Since we're using Vortex Showcase under the covers, you can also upload modlists with only some mods. Select multiple mods in your Mods list and then click the Create Showcase button at the bottom of the Mods list. Optionally, enter a title and make sure to choose the Modwat.ch format when prompted. One the showcase has been generated, you should see a notification with a Publish button. Click that (and enter your username/password when prompted) to immediately upload a modlist using just the mods you had selected.