This might seem super-obvious, but I’ve made the mistake several times of assuming that clients understood it: the total number of pages in a saddle-stitched booklet need to be in multiples of four. In other words, using your situation, open the 8.5x11-inch booklet and it measures 17x11 inches. There are two pages on the front and two pages on the back of each sheet of 17x11-inch paper. In still other words, a saddle-stitched booklet can have 8, 12, 16, 20, 24 pages, etc., but it can’t have 7 or 13 or 18 or 37 etc.
Another hint: sometimes it’s useful to create a little dummy booklet to keep track of the pages. Just get a few sheets of 8.5x11-inch paper, put them together and fold them in two to make a little booklet. Now you can mark the cover, the inside cover, page 1, page 2, etc., all the way back to the inside back cover and the back cover. Sometimes it’s useful to have a physical sample to pick up, look at, thumb through to keep track of which pages go where, where the middle spread is, etc.
Another thing: odd-numbered pages are always on the right. Even numbered pages are always on the left.