Should I use the sample doc as a template or my existing Pages doc?

This is my first attempt at creating an ebook. I want to publish a novel I wrote on iTunes. I was experimenting with iBooks Author but found out readers could not change the font or point size with IBA and I certainly want them to be able to do that so I'd like to make it an ePub. My book is text only and I was wondering which is the best route: use the sample doc as a template or my existing Pages doc?

And (I'm not sure yet if this is dependent on the method I choose) should I single or double-space my text?

And how do I add a graphic/artwork to the cover?

Any help will be greatly appreciated, thank you.