You should have your document in pages configured so that the PAPER size is half the landscape size of what you are printing on. i.e:
Letter Booklet = 11x8.5 paper so use 5.5x8.5 paper size when making your document
Legal Booklet = 14x8.5 paper so use 7x8.5 paper size in your document.
I ususally set 0.5 inch margins all around and turn off headers and footers (unless using them for page numbers)
Print to a PDF
Open that PDF in BookletCreator, make sure sheet size is correct and save to a new file
Open that file in preview and make sure it looks correct.
Printing can be a little tricky:
Many inkjet printers (HP deskjets for example) cannot print down within 0.5 inch of the bottom of the paper. HP drivers for example rescale the document to fit in the printable area even if you set size at 100%, so the middle of the sheets come out off center and all the margins are wrong. I haven't found a way around this problem other than to use a laser or a printer with gutenprint drivers.
I think Adobe reader can sometimes override this but it's tricky and requires experimentation. Some nicer printer drivers can make the booklet automatically.
By the way: APPLE, WHY ISN'T THIS BUILT IN????