Thankee, muchly !
For those who'd like to try their hand, use brown paper shopping bags as a pattern, first.
Cut out a piece for a front panel , a back panel 2X as long, and a 4" or 5" wide very long wrap-around strip to use as a side gusset, and two large tabs for sewing to the bag top or gusset as ears to hold the strap ends.
(This bag has a double front panel, the outer cut a tad shorter than the inner, to form the full-width outside front bag pocket after sewing, see below)
(This bag also has a full-width, rectangular panel, to be sewn to the back panel at both ends, the bottom, and up the middel to form two inside pockets)
The strap can be cut & fitted to the bag ears, with a buckle, later.
Punch holes in the leather for heavy, waxed thread, along the edges (proposed seams) and where you intend to add internal features like pockets and/or strap anchors.
Sew the pockets, strap anchors, and other such to the inside and outside of the front and back panel, etc - also first doing any leather decorating, like fancy stitching, fringe, carving or stamping.
The front and rear panels are each sewn, in turn, to the gusset, with the back inside-out.
Leave the extended back piece for the front flag alone, to shape & finish off later.
After the body of the bag is sewn, and optionally riveted at the top ends of the sewing, turn the bag right-side-out.
Slightly dampening the leather helps.
Now when you can view the nearly completed bag, is the time to shape/trim the flap and setup a closure - usually a rawhide loop over an acorn or somesuch.
Measure the length, over your shoulder, opposite the side the bag's to hang upon (wifey-poo's long strapped purse is good for this), and cut or weave a strap at least 12" overlong that measurement.
The strap can be either a 2-piece, w/buckle, or a one-piece, per your druthers.