16 pages zine from single A3 double sided printed sheet

Here is a 16 page imposition guide.

From the back fold the sheet in half so page 2 lines faces page 3, then flip the sheet and fold down so page 5 faces page 4, then fold again so page 8 and 9 face each other. Then with page 1 as the cover, you would trim the top and right edges, with the spine on the left.

Take a blank A3. Fold it (when starting landscape, fold left over right, top over bottom, left ovrr right). Add numbering - take care to draw numbers in the correct orientation. Unfold, and you'll see the order and orientation of each "page".

It should be 1-4-5-8-9-12-13-16 on one side, and 2-3-6-7-10-11-14-15 on the other side.

Only do this if you're printing it yourself (and your printer is exact enough!). If this is going to a pro print-shop, design your pages in their actual – much smaller – size and let them handle imposition.

The easiest way to tackle this would be to fold a blank sheet of paper into the final format, number the pages and then unfold it again.