Hi Luke,
A very good question.
Firstly, have a look at this site, it will give you the knots (
www.netknots.com)
Now to the specific question.
The amount of backing depends on a couple of things. Firstly what size is the reel? What is the line rating? How far should I fill it?
Let's start with the last. The amount of backing, plus the line should fill the spool but not overfill it so that it jams when reeling the line back onto the spool. I would say keep it all just under the rim.
Now the other tow. It is easy for us experience people to give estimations etc but difficult for a newcomer. So here is my suggestion for you.
Wind the fly line onto the reel and once you have got it all on, then attach the backing and keep winding until you think you have got it right.
Once you have done that, reverse it and you have done the job.
But as in all suggestions, there are a couple of hitched. If it is a double taper line, then you are okay. If it is a weight forward, you will need to make sure you attached the backing to the right end. You should be okay as most lines have a little tag to tell you. The other bit is that you will need to lay the line and backing out once you pull it off so you can put it on the right way without tangling.
The good part is you will only need to do it once.
Others will give you a lots of good suggestions. This is how I did it my first time.