I agree with Squirrelly-ORC; follow Gilmore of OK's advice and watch those video tutorials first. There's no quick and easy way to learn how to do it, it takes some time doing research into the process and reading/watching whatever you can find online. Those of us with experience in molding making and casting can help with any questions you may have
From personal experience, any and all of the mold masters I've scratch-built were made from various forms of plastic (layered sheet styrene, Sintra (PVC) board, etc.), which when complete I then primered before putting into a mold box. I typically use cut up cardboard for the walls of the mold box, which I hot glue onto a flat, level surface. I make sure to thoroughly glue all the seams (bottom of the walls, inside and outside of corners) to avoid any silicone leaking out.
One that's done, I apply a little bit of glue to the mold master to firmly secure it inside the mold box, then after applying a mold release agent to the inside of the mold box walls and doing a little bit of basic math to figure out the volume of silicone needed to make the mold (mixed at a 1:10 ratio -- 1 part catalyst to 10 parts silicone) I pour the mixed silicone into the mold box and let it cure for 18-24 hours. I usually leave it for a full 24 hours just to be on the safe side.
I myself have always done one-piece, open-backed molds for the props I've kitted out, meaning the entire back end of the prop is the pour spout as all of the detail has only ever been on one side of the piece. In your case, the process will differ slightly as you'll be making a two-part mold with specified pour spouts to make sure you get all the detail on both sides of the prop weapon you will be molding and casting.
The specific process for what you'll be trying to achieve can be learned from the tutorials linked to in this thread and found elsewhere on the 405th.