It's all out on show, there's no hiding anything I'm afraid! I'm always told to put my knees pointing outwards so they can get to the sides then my knees up to my chest so they can get underneath. It's not really that embarrassing though because they do it every day and have seen it all before!
They usually give you disposable knickers (which I hate so I wear a black thong that I don't mind if it gets wax on it) and use these to keep the skin taut while they do one side at a time.
They do it all day long and at least its less visible than when you're getting a smear!
P.S: If you're worried about pain, make sure you are well hydrated, have eaten within the last two hours and aren't on or about to be on your period