The first most important factor in making a great holster is having a design in mind when you are making it. If you design on the fly, or as you go, you will be correcting mistakes all the time. Each holster I make has a drawn out design, patterns made from that design and then I start to cut leather. Your design should also include:
location of stitching
Location of tooling or other decorative design
location of any hardware that might be used
Once you have the design, next you will need to identify all the steps necessary to complete that design. Every holster will have it's own set of steps, and each leather workers seems to have a slight variation to the steps, none of them are wrong, But some are more right than others. My more disappointing mistakes have been a result to rushing the project and not taking the time to follow my steps as I had planned.
My (basic) steps
Design and make patterns
Cut and skive leather to desired thickness
Double check the fit of the gun to the leather, ensure enough room for stitching
Using pattern, mark, punch holes for stitching
Stamp/tool design if any
Dye to desired color
Finish straight edges that would be difficult to finish once sewn.
Sew, making sure to sew multiple parts in correct order so that all parts have the same strength integrity.
Mold the leather to the gun.
Finish edges.
Apply a finish to the whole project (if desired.)
In the past I have had to re-sew some projects because before I realized what I was doing I was sewing something out of order or didn't complete a prior step.
Hand Sewing
Machine sewing pushes a loop through each hole, which is caught by the Bobbin on the other side. this create a link between the two materials and if the thread were to wear out and break your seam would come undone quickly. When hand sewing, I hand punch the holes. I take a single piece of thread and put a needle on each end. Using the middle of the thread as my starting point each needle passes from opposite directions through each hole. Hand sewn thread is normally thicker than machine thread. It is also waxed, this gives it much more strength than machine thread, if thread is somehow broken there is still a complete thread on the other side holding the seam together.