I started making holsters in 1972, and I have made many thousands over the years. Pattern making is an integral part of each project. Learning how to locate stitch-lines so as to achieve a proper fit is part of the learning curve. Selecting materials appropriate to each project requires some experience. Forming and finishing a holster requires some knowledge and experience.
Much of this experience is gained through trial and error. I think that the vast majority of holster makers will freely tell you just how many "chew toys" they made before they achieved a result worth using.
Last year I developed two new holster designs. From the original prototypes, through testing by experienced users, to the first marketable product required 3 to 5 months for each design, consumed several hundred dollars in materials, and required many frustrating hours between the occasional "OK, that works!" moments.
Imitating or duplicating the designs of established holster makers is fine for your own use, while doing so for profit is ethically wrong. You may learn quite a bit by trying to make an established design that you like.
Creating an entirely new design is an involved process that I would recommend only for those who have achieved a level of experience that permits some chance of success.
Either way, you can expect to spend a great deal of time and a fair amount of money on supplies and tools before you have a finished product that meets your needs.
I am not trying to talk you out of trying. I encourage anyone with a desire to make their own to do so. But you will need to have more than average patience and persistance to achieve your immediate goal.
Best regards.