One of the holster manufacturers (philster / PhillyEDC I think) had a very good YouTube series on how to make a good kydex holster. He went into a lot of detail on time/temperature, tools used, etc. while you don't necessarily need all of the tools he uses, they do make it much easier and faster to make a holster.
For a press, I use a standard (basic) sandwich type press. I have a square piece of plywood (maybe about 18-20" per side) with a 2x4 across the middle, and foam underneath the plywood. Get good foam - it's not expensive and it works better than crappy foam pads. I use 2 clamps (more pressure is better, to a point) to hold the two sides of the press together.
I would highly recommend getting a dedicated molding prop instead of using a real gun. For one, there are places where you will want relief beyond what is on a standard gun or blue gun so that you don't have extra retention beyond what you need. For example, you don't want things like takedown levers, rail slots, and ejection ports to be holding the gun in the holster. If you use a regular gun or blue gun, you'll have to add in your own blocking to provide relief at these areas. A good molding prop should have these already built in. If you decide to use a (real) polymer gun, stick a magazine in the mag well (it will help support the grip). Between the heat and pressure involved in the press, it's not unusual to hear of polymer grips getting bent.
One of the best parts of making holsters is R&D of different designs to figure out what works best for you. Your largest raw material cost will likely be the belt attachments, but you can reuse those while you're learning. If you don't count your time or "machinery", your cost for each holster body will probably be something like $5, so keep trying different things. Find some holsters you like (that other companies are selling) and see if you can copy their design. Once you get good at making your own holsters, you'll never want to pay $100+ for a holster again.
One thing that I have found is adjustable retention holsters are easier (for me) to make than non-adjustable retention (especially for a taco style holster). Add a tad bit of extra blocking and then use some fuel line (from an auto parts store) to provide the outward pressure, and a screw to adjust tension. It's much easier to let the end user find the perfect retention, and it can be a selling point.
