Welcome to the forums!
A picture reference is very helpful. I'm linking one below:
It looks like it's a pretty standard shoulder holster. One method (easiest, IMO) is to just go out and purchase one. You can find them easily online, and from a quick ebay search, most will run you between $30-60 (leather is usually more expensive, nylon is on the cheaper side)
If you want to make it yourself, you'll need to gather materials to make each part: The actual holster, the straps to go around your shoulders, and other bits. You can try making them with actual leather, but it'd probably be cheaper and less work to buy a leather holster yourself.
For my Strider Hiryu harness I made the straps out of craft foam, and covered them in a faux leather/vinyl fabric.
The same principles could be used for your holster. Just get reference of the holster and draft out all the parts, cut them out of foam, and cover in fabric. You can even use nylon webbing and actual buckles to make it more realistic.