Like, how do I add text and images to them?
You go to the template page, click "Edit source" and add what you want. But it also depends on the template type.
Hm, okay. And images?
The same.syntax as on a page, [[File:IMAGE NAME]], see Help:Adding_images
Again, it also depends on the template type. Infoboxes in particular have special syntax.
We could help you more if you gave a link to the specific template you want to edit... it sounds like you're talking about something that doesn't exist yet.
What do you think?