Summarize:

Use a bevel tool to cut the square off of the edge of your leather. This will keep the edges from rolling up over time. It is only necessary to bevel one side of the leather: the back side can remain flat. Pay special attention to any sharp corners on your leather piece. These will need to be beveled separately, creating a diagonal or cut corner. With your edges beveled, you can move on to adding whatever finish is appropriate for your leather. You can use a resist to keep it a particular area clear of the finish or to reduce the coloring. Just be sure to always use gloves when using dyes or finishes....unless you like your hands a different color!  Tandy sells excellent finishes in a variety of colors. To apply these, use a paper towel, wool dauber, or a natural or very old sponge (regular sponges contain a chemical that does not react well with the leather). You can also use dyes. There are dyes intended for leather, but some leathers can also be dyed with shoe polish! These should be applied in the same way as the finishes. Acrylic paint thinned with water works on leather, if you'd prefer a painted look. Just use a normal paint brush to apply it. This works better for smaller, detailed sections than larger ones. You'll want to smooth the beveled edges of your piece after dying it. You can use a special tool for this purpose but the cheaper option is to simply rub it with a paper towel or canvas cloth! This will make the edges look shiny and professional. If you plan on doing any stitching, you'll want to add a stitching groove to keep your stitches from being worn down over time. There is a tool for this purpose called a stitch groover. Once you've cut a channel into your leather where the stitching will go, use a pricking iron or overstitch wheel to lightly mark where your individual stitches will go. This will help create an even, professional look. Finally, punch the individual holes you marked using a piercing awl.  When buying an overstitch wheel or pricking iron, get a shorter one to start off with, as it will be more versatile and make it easier to turn corners. When starting a new stitching line or having to otherwise pick up the overstitch wheel to start again, place the first spoke down in the hole of the last section of the line. This will keep the stitches even. Before sewing, you'll want to add any other finishing touches that you know you'll need, like adding snaps. Be aware, however, that adding things like snaps is difficult and requires its own separate set of tools. Once that's done, add your stitching and you're basically done. Rub the whole surface with a cloth to get a nice, smooth final look. As a final measure, you can add a clear final finish on after doing your stitching, if you want to. This is not necessary, however. Enjoy your new creation and don't forget to keep practicing!
Bevel the edges of your leather. Add finishes, dyes, or paint. Smooth the edges. Punch holes for stitching. Add other details. Add a clear finish if desired.