I started by cutting 2 pieces of book board: you need to allow 5mm more than your folded paper inner all round. I then cut 2 pieces of book cloth, allowing about 20mm overlap on three sides, but cutting it 20mm short on the spine edge. You stick the book cloth pieces to the boards (it's easier to apply the glue to the board, rather than the book cloth):
Then you do the endpapers. You would normally stick the paper to both sides but, in this case, the inside back cover is going to be completely obscured by the flap we need to make, to slip the paper insert into. You just need to cover the front. Measure a piece that is 4 - 5mm smaller than the front cover all round and stick it on.
You're nearly there now. I'll tell you how to make the card insert next time, as this is turning into a very long post.