Welcome Mac!

First of all, drill down to where your site should live - I guess http://www.themodernantiquarian.com/site/3187/northumberland.html in this case. Now, under the web searches bit of the page, there's a dropdown -Add- and almost at the bottom you can choose to add a new site. Name it, choose its type and add grid references, and you're off, ready to add your images and fieldnotes in just the same way.

I'm sure you have done already, but make sure to read the guidelines on submissions!

Sals x