Notice how each button has source - local or source - steam written under it in fine print. The local ones are in the mods/ subfolder where you can work on them.
If you’re successfully connected to Steam, each of these local mods should have an Upload to Steam button displayed.
Hit the button, hit Request ID, fill in the form and then hit Upload Settings and Content, and you’re done.
It’s perfectly fine to download your mod from steam as well in order to test it. This shows up as a second identical mod labelled ‘source - steam’.
If you want to make an update, it’s just like the initial upload except you use the existing Workshop ID number you requested, instead of asking for a new one.