If nothing else, these posts will put you on the right track. Brokennock's example being of great assistance. Before you start cutting up the leather, though, make sure you take stock of what you are using this bag for. I can only speak for trail walks, but I ended up with a small belt bag that holds anything to do with cleaning the bore and getting back into the car.
The reason for that was it was easier getting into a small gun bag for each round and only getting into the cleaning bag every five or six rounds or so.
Like a lot of guys on this forum, I've been banging around for 40 years trying to make the perfect bag, the perfect rifle, the perfect shoe or moccasin and so on.
I finally settled on two;
This guy ended up being my second, smaller bag;
They are the last bags I will ever make....for now...