Here is another peek into my sewn journal (see other posts HERE and HERE)
I had this wonderful old german book with a leather spine - I made 3 signatures the size of the book:
I then sewed them together onto book binding tape.
Using glue, I adhered the signatures to the inside of the spine.
And I added some pretty tape over the edge on the first page to cover up any unevenness.
And the leather spine stayed intact and my pages are securely binded in.