Hit 'T' to view your schematics. On each schematic, there's is a downwards arrow that sort of looks like a "download" icon, second icon from the left. Click that, and then click Share on Workshop

also press and hold F to copy buildings. It'll then give you the option on the bottom to save schematic. If you wanna find them it's the small clipboard icon on the building page.

Originally posted by Eudae:

Hit 'T' to view your schematics. On each schematic, there's is a downwards arrow that sort of looks like a "download" icon, second icon from the left. Click that, and then click Share on Workshop

Thank you very much! But it tells me now that my content could only be seen by me. Do you know maybe why that is?

I think there is a privacy toggle somewhere on workshop items, set it to "public". If that's not it, I am unsure.

it means only you can see it

if you want to change it to friends only public or private there's the option in the workshop page for the schematic.