Al Stohlman's book How to Make Holsters gives you all the basics for several types of holsters. It's available on Ebay, amazon et.
There are dozens of youtube creators that have produced how-to vids on holsters. Some are detailed and excellent quality, others, not so much. Tip #1, be sure to stick with veg tan leather, especially next to the firearm. Chrome tan can cause corrosion. Be careful of the youtube vids, there is at least 1 guy who promotes using chrome to line holsters. Don't do that!
When I first reentered leather craft about 20 or so years ago, my first project was a Slim Jim holster for my 1858 New Model Army BP revolver. I used some vague written directions I found on the web somewhere, incorporated some vague memories on how to do things from my childhood leathercraft days, and built a holster that fits, functions and looks just fine, but which I know now was done all wrong. IMO, if it works, it ain't wrong.