I looked at Jimjams design and made this from an A3 Sheet of card in the same way.
Book Measure nearly 6" by 6".
But I cursed several times and had to go back to look at her pattern to see how to fold it.
To upsize it to any size paper you just need to score the cardstock in half.
Then score 1 1/2" at the bottom of the length and divide the rest by 3
A3 scoring Guide.
11 7/10" x 16 1/2"
Score at 1 1/2" , 6 1/2", 11 1/2"
Score at 5 17/20"
(your really just scoring on the A4 Half-fold line)
I did cheat and added covers from A4 cardstock folded in half and scored at same wide as my pages.
These were tucked into the middle fold as you can see in the photo below.
I used a pack of freebie 6 by 6 papers that I got off a magazine last year.
I added ribbon in the same way as the instruction sheet said to hold it together.
Here's the back of my book.
I added stickers of the puffy kind
that I found in "the works".
Got loads left to make some cards
or tags to match.
Have added the link again to the great tutorial.