there is no specific hole to a vagina, that's where it gets confusing, so you know where you pee from and where you poop from? it's in the middle and slightly towards the back as you say. Now as I say there isnt a specific "hole" as such, you need to open the area up with one hand and insert the tampon with the other,sometimes you might have to fiddle around to find where the actual opening is and if you havent broken your hymen
yet there will be a specific area where there is a gap to let your period out of that will allow you to push it in. it says "aim towards your back", it just means in a basic upwards motion but at a slight angle towards your back. I was always nervous about using tampons but I found that it was much easier once I knew where everything was down there and was comfortable with it. you know it's worked when it slides in easily and you can't feel it when you stand up (or walk if you are standing already). If it is uncomfortable you may have inserted it incorrectly or not far enough in. It may take from 2 goes to 10, just depends on how nervous you are, so don't be downhearted if you don't get it the first couple of times.. If you find you are getting all tense your muscles down there will contract so hard that it won;t let anything inside, so take a break and try a bit later. if your periods are very light make sure you use a very light absorbancy tampon a) they will be easier to insert and b) be easier to take out again.