Plaster comes in quality ranges. The plaster sold for slip casting is a high quality and, no doubt the site or store will have an answer for the beginner.
That said, all moisture must be out or near the outer surface of the mold before pouring your metal. Part of you problem may be that you have a one piece mold. A two piece mold allows the surface area you wish to reproduce to release moisture and dry when opened to the air, whereas a one piece contains an atmosphere that is usually more populated with water molecules than the outer surface, so the path of least resistance will be the outer surface and water will try to find it and remain in the plaster.
Your oven should be adequate to the task. Heat it to 350 or whatever the site or store recommends and expect to leave it there for hours, depending on the density and size. Also, be sure the mold is stabilized with the opening facing up to permit quick venting.
When pouring your metal into your very dry mold, venting will again have a roll. Picture water shooting from a hose and then picture that hose shooting water into a glass pitcher. Your molten metal will act like the water when it hits the bottom of the pitcher. You want as little of that as possible!
To help prevent the turbulence vents are added. The vents are created by drilling a few small diameter holes directly through the image surface and into the region that will be filled with tin. Gases, including water vapor, which also cause turbulence, have an escape path and will leave just before the hot metal does. The metal will find them and the escape of some metal will reduce the problem. These vents need to be chased off, after the mold is removed. Soft metals are easy to deal with and tin falls into that category. Hack saw, files, or sand paper should help hide the blemishes created by the removal.