Would be interested in how this works, since I have a good bit of spent brass around.
first obtain your brass, a hole punch, needle nose pliers and a small ball pein / tack hammer:
punch your hole in your leather with the hole punch (the 6mm one is a good size). pop the brass in. flay out the edges gently by pushing the nose of the needle nose in the hole. i have a tiny ball pein the size of a tack hammer, but a large ball bearing would work too. gently tap out the edges until rounded over.
that my method, nothing too fancy but seems to serve. if you got lots and some scrap leather play about with it. if the casing collapses flay out using the needle nose pliers more.