One of the things I like about the kind of leather journal/sketchbooks I make is that there are several ways to use the wrap-around strap to keep a journal closed, depending on how you want to use it.
One way is to simply wrap the strap around the journal...
....and on the back side of the journal, tuck the end of the strap under the length that wraps around.
I tend to be a bit of a magpie when I travel, picking up ticket stubs, postcards, pamphlets, etc. Because of the way my journals are sewn, there's room for expansion, so you can store lots of those irresistible little items in between the journal pages.
I make my journals with straps that are long enough to still go around the journal even when it's gotten quite fat. Here is one way to tie a knot in the strap to keep the journal snugly closed:
This is one of my little pocket sized leather notebooks. You can keep it closed by tucking the end of the strap under the rest of the strap from above:
To make it even more secure, after you tuck the end of the strap under, loop it around and tuck it under a second time: