SLOWLY, or it will crack, warp, and/or check. While checking isn't usually a problem for structural wood, It's not usually the look people are after for a mantel.
Cut the tree, leave the bark on. Dry the wood slowly by moving it over the course of weeks into a climate controlled area. If you're in a humid area, it'll take longer to dry. Maintain temperature, and decrease median humidity level over time, and then raise temp slowly. review woodworking websites & references for step-by-step instructions, but that's the gist.