A leather strap closure to wrap a book or portfolio together is something I use all the time, on paper or leather covers. It is fairly easy to do and dates back to the 2nd century CE. Here are the steps, illustrated with photos:
Cut three slits in your cover paper, whichever side will be overlapping the other half of the front cover; the slits should be just slightly larger than the width of your leather strap. Cut both ends of your leather strap to be pointed. One one end, cut a slit about the width of your strap, at least 1/4 inch from the end of the point. Thread this end from the outside to the inside in the first hole, closest to the spine until the slit opening is fully through to the back.
Thread the long end of the strap from the outside to the inside through the second, or middle, slit, then thread it through the slit in the other end of the strap.
Then adjust the two to be as flat as possible, snuggling them down to the cover.
Finally, thread the long end of the strap out through the last slit closest to the edge of the front cover, to the outside of the cover. Use the long strap to wrap the book at least one or as many as three times, and tuck the end in to make a secure closure.
This is the finished portfolio, a scrap piece of paste paper I was using to practice some gilding with faux silver, and not exactly happy with the results, still saved it (never throw paste paper away, you never know when you might be able to reuse it, repaste it, or cut it up for collage projects). Later I plucked this piece out of the flat files and decided to turn into a long thin four-fold portfolio (also a form I use often) for some special scraps of paper from books made in the past, some of which have gone to other homes now.
Here is one more view of this type of closure from the back, using a much narrower leather thong on a soft suede leather cover. For more views of this book, see A Goddess Alphabet in the gallery.