Regardless of the program used to format the covers, it boils down to the following steps:
1.- Start with a Legal Size Page (8.5" X 14"), landscape mode without margins.

2.- Measure from Right to left and create 3 sections on that page.

6.- The right and left sections will measure 5.5 x 8.5”
7.- The center section will be the spine, and the width of that section will be found by multiplying the number of pages 0.002 (example 200 pages x 0.002 = 0.4)

Include within those sections whatever you want your cover to have.

Then print the page into the InstaBook PDF Printer, or Select all and Export it as a graphic .jpg