The rim is mounted to a wooden plate that is attached to a hub that chucks into the lathe. The rim is mounted to the plate with screws. It needs to be mounted evenly so that it will come out as round as possible. There is some danger of cutting through the outermost layer if the rim is mounted unevenly. Go slowly, trim the outside of the rim as well as the inside, let the lathe do the work and watch out for tear out in the case of figured woods. Take off as little as possible to get the rim to the desired size and shape. To help center the rim, make a mark on the mounting plate that matches the circumference of the  outside of the rim. The mark can also be used to inform you of when you have gone far enough. While you have it spinning on the lathe take some sand paper and smooth it up. In the case you have no lathe, the rim can be trued somewhat using a combination of the block plane, the file, and the power sander. For safety and success, a live demonstration by an experienced wood working professional is a must for beginners at lathe work.