I've noticed a lot of people complain about how bad it is, but it's actually very simple. Once you've gone to advanced search, the first thing you want to do is click the tab that says "search single content type." From there you can obviously input whatever you want to find into "keywords" though it won't search words that are three letters or less. Then you can add in a username if you're looking for something from a particular person. After that, scroll to the bottom where it says "show results as" and change it from threads to posts.
Edit: Ugh, I feel like this sounds so condescending and that was not my intention.