My tips:
1. Just get started and start out quick and dirty. You don't need anything other than a toaster oven, some hard foam, a few big heavy books or two pieces of scrap wood, sandpaper (heavy and light grit), a saw, a craft knife, a drill, and a heat gun.
2. Think about your design first. Wraparound design is easier than two piece, especially if you are only going to have a single clip. Expect failures. That's fine - you'll still end up ultimately producing a great holster for a fraction of the cost.
3. Plan for exactly which part(s) of the gun are going to index into the holster to make it "click" for retention. The trigger guard is typical, but may not be the best part.
4. When you cut and break a section of Kydex, you'll see white along the break line. If you are able to watch that line as the piece heats up, you'll know the piece is ready for molding when the white turns black.
5. Don't get too wrapped up in riveting. It's actually not that necessary. I've done a bunch of holsters without any rivets...and quickly realized that with the right design you don't need them at all.
6. Use clips like the Quick Clip Pro that have now pretty much standardized on the distance between screws. This will make it a lot easier on you if you want to add things like claws, or swap out other clips.
I can slap together a holster in under an hour at this point for about $12 (most of the cost is the clip, which can be close to $10) for a design that many manufacturers are selling for $70+.
I cut a sheet, put it in the toaster for a minute, remove it and wrap the gun, put it in between two foam sheets, throw a large book on top and stand on it for a minute. Then I drill screw holes, make relief cuts to get the general shape of the holster, sand all edges, put the gun back in and use a heat gun to dimple the trigger guard to ensure the right level of retention. Add the screws and clip and I'm done.