How do I do this wordmark like the adopt me wiki?
Do you mean how to create the picture, or how to upload it?
The picture is a piece of artwork, we can't really help with that.
Once you have a picture, you can upload it via Theme Designer
Note that the layout of ThemeDesigner is different between the old Oasis skin and the new FandomDesktop skin. However, you must upload the image separately to both; assuming you want it on both.
What do you think?