Well this is the way I would do it. I'd go to the 'Map Browser' and stick in the grid reference so I could see if it had already been posted up (perhaps under a different name). Your Cnoc Aine at R677375 isn't there I see.

As it's totally new, I might would see what area nearby sites had been put into (maybe in Ireland these are mostly counties so it's easier, but supposing it wasn't in Ireland..). A nearby site is 'The Stone of the Tree' - so click on that on the map. Then click on the name when it comes up on the right.

When its own page comes up you can see the 'nesting' along the yellow band near the top of the page. In this case it says Limerick > Grange / Lios, Lough Gur

Probably you'd want to put it in Limerick, rather than the grange subsite??
So click on 'Limerick' and you'll see all the sites in Limerick. You'll see the 'Add' box on the right. Click and scroll down to 'Add a site' and then you'll get a screen to add the location, name, type etc (I think this bit is probably self-explanatory when you get there).

Once you've got your site you can add fieldnotes, photos etc.

Clear as mud probably.

You might want to read the submission guidelines too at
http://www.themodernantiquarian.com/submission_guidelines/