It's relatively easily done if you're adding several sites in succession, so you just need to return to the main county page after each one. Look out for the message 'You're adding a site to 'X', are you sure you don't mean to add it to 'County Y'?', which should come up if you try to add a subsite. It's a learning process so don't worry, and the Eds are silently tidying where they spot anomalies.
Thanks,
TMA Ed.