This is my finished product. I follow a particular set of steps for all of my projects.

1 Identify my needs and constraints

In this case it was to quickly produce a shoulder bag that could carry 2 eyeglass cases and a notebook.
I didn't have time to stitch a bag so I was to use only rivets and other hardware
I also did not have time to treat a veggie tanned leather so I used a precut chrome tanned leather. Chrome tanned leather is often used for these types of projects because they are pretreated and more flexible for bags and clothes.

2 Plan, Research, and Design

Image searches, leather suppliers, rough sketch

3 Create a template

Using the desired leather piece, 12x24 precut, I used Illustrator to design the template
Using a CriCut machine I cut the template out of cardstock
Folded template and assembled with removeable tape to verify holes and lines were placed correctly
Make any necessary changes. Some changes like the placement of the snap and the placement of the rivets holding the flap I waited until using the leather instead of pre-punching holes

4 Assemble

Place the template on the leather and mark cuts and holes
Cut leather
Treat with dye if needed
Assemble using correct hardware
Perform finishing touches

5 Finish

Step 1: Tools Needed

If you use chrome tanned leather like I did you will need a hole punch, rivet and snap setter (not pictured here), and a strap cutter. If you use veggie tanned leather you will need to dye the leather and treat the edges with the shown optional tools. You will also need a way to cut the leather. So when using chrome tanned, which at this point I would recommend you will need the following:

hole punch, hand held like pictured, or one you put in a press, or use a hammer with. Make sure the hole is large enough to put the shaft of the rivet through but not so large you risk the base being pulled through.
Sharp knife
Rivet setter, hand held like pictured, or one you put in a press, or use a hammer with. Make sure the setter is the correct size for the rivets or you risk distorting the head of the rivet or bending the shaft
Snap setter, hand held like pictured, or one you put in a press, or use a hammer with. Again make sure the size of the snap fits the setter. You can use 20 or 24 size snaps
Strap cutter if you so not have pre-cut straps

Go to Tandy Leather, Springfield Leather (I like their precut leathers), or the supplier of your choice. Search for the rivets and snaps. Then look for rivet setters and snap setters. You should find the three options listed above and choose the one that works best for your needs. Get the same size as the rivets and snaps.

Step 2: The Rough Sketch

For this piece I wanted a simple, good looking leather bag, that was as small as I could make it but being large enough to carry what I needed. I also needed it to be quick and easy to assemble. So I started with the idea of using a pre-cut piece of leather that was 12x24. Using my notebook, I started to sketch my ideas with rough dimensions.

Step 3: Materials Needed

Rivets, pictured, enough for the size of your bag. Figure about an inch between rivets up the sides, however many you want for the top (I put 3 per side for decoration), then 6-8 more for the strap. 3 per side and 2 more if needed to put two shorter straps together to form a longer one. This is what I did.

1 snap

2 D-rings or triangle rings like I used

2 clasps to attach the strap to if you do not want it permanent. D-rings or square ones are better if you do not use a clasp and just want to permanently rivet the strap to the bag

1 piece of leather large enough for your bag. Mine was 20x24

1 strap. I cut 2 smaller straps from an extra 12x24 and attached them together to form the full strap.

Step 4: Creating the Template

Once you know the size of the leather for your bag, the number and location of the holes for the rivets and snaps, you can create a template used to mark where to cut the leather and punch the holes. I created mine in Adobe Illustrator and then used a Cricut machine to cut the template out of cardstock. You can do the entire thing by hand with a ruler and pencil.

The first image is the design I used. I then cut the design out of card stock.

The second image verifies that the items I needed to put in the bag would fit.

The third image shows the assembled with tape prototype. I put in one rivet to verify the size of the holes. I then made sure that all the holes lined up.

The last image shows the now disassembled template clipped to my 12x24 leather with the awl I used to mark the leather for cutting and punching.

Step 5: Assembly

Pictures 1-3 show the process to rivet leather together. First punch a hole in each piece you want to join. Next insert the stem part of the rivet through the holes and attach the cap. Join the rivets using your preferred method.

Note that the press gives the most consistent results but is the most expensive. It is also the easiest to use and the quickest. I recommend getting a press if you are planning on become a leather worker. If you only plan on random projects you can get a hand-held setter that looks like the hole punch. If you are willing to take a little more time and want versatility, get the tap setters you use a hammer with. Many professionals use these but they are harder to get consistent results with and this project would be difficult because you are setting rivets in the narrow sides of the bag.

Step 6: Assembly Cont.

The first image shows the bottom sides. Here you need to go through three levels of leather. The bottom and both sides. The next two show how to rivet the strap. The ring is attached with a short strap that one side has attached to the bag using one of the rivets a few down from the top. The strap is then fed through the ring and riveted together. This is the same technique used to attach the clasp to the shoulder strap.

The top is a special case because I realized I could not precut the holes as it would be better to fold them accurately so the holes lined up. If you look at the middle picture it shows how I attached them together I folded the top down and then folded the front over it. I used a line of rivets both for decoration and to hold the inside and outside corners of the folded flaps tight.

NOTE: you want to fold the leather so that the fold is facing front. This is easiest to see in the middle picture. The front of the bag is on the top so the flap opening is towards the back. This gives a cleaner look. Also you want to make sure that the rivets are assembled so the decorative cap is facing out.

Step 7: Lastly the Snap

Once everything is assembled you can put the top over and find the best placement for the snap. I put a small piece of wood on the inside, lined up the top so that it was far enough down and centered. I then used an awl to poke a hole directly through both pieces of leather.

Punch the two holes and attach the snap. The technique is the same as setting the rivets but you will need a special attachment or tool for the snaps.

Step 8: Miscelaneous

The strap cutters are used to cut a consistent width from a wider piece of leather. You can set your desired size then run that down the leather to cut your strap.

As an FYI for anyone interested in the notebook cover. You can attach notebooks two ways with this design. First you can put the flaps of the notebook under/between the cover and the white support board. Two you can use the cord threaded through the spine to go into the middle of the notebook.