For this project I used an old Leather Coach Willis Bag, but any bag with top rings will work.
My bag was missing it's crossbody strap. With this alteration technique it can be worn as either a crossbody (above) , or a backpack (below).
I attached a jump ring, in a rectangle shape to the bottom of the bag. You'll need a piece of scrap leather and a rivet. I got the jump ring from another old bag, Re-purpose as much as you can on this project .
At the bottom I'll have a supplies resource list.
I inserted the hole needed for the rivet through and between the stitches at the bottom of the bag. The rivet can be removed later if desired, and the bag will return to it's prior condition. I used an awl to coax the stitches apart just enough to insert the rivet shaft. (you could also use a small screwdriver to coax the stitches apart).
I used a couple of old belts (again re-purposing) to make a long strap that can be separated using head button studs to fit it to the size you desire. I placed the studs 2" apart on one belt and punched holes 2" apart on the other.
I attached the straps to the top rings with rivets, this could also be done with the Ball Studs to make it fully detachable.
You'll need some specialized equipment to do this. Here are links to the items used via