How do i create a channel?

Okay, there are a couple of ways to approach this but this kind of work is very specific, so it could take a couple of tries.You will want to practice up abit before you tackle your fine metal [silver/gold/platinum]. If you know a machinist with a small scale mill, you might be able to take that direction. The mill would have some sort of clamp for the ring and a machining head or 'tool' to do the cutting. Most of the machinists I know are quite particular about specifications, for example - "I want a channel that is v shaped with a depth of 1.5 mm and a width of 2mm and an over hanging lip of .165mm".
Next method would be the tools jewelers use. There are ring clamps as well as small scale vises that can be used. A tool called a flex shaft has fairly high rpm and you can fit bits into it. The burs that you probably would want to use are called Hart burs and come in different angles, you may also consider bearing cutter burs. Usually a ring rough [called a shank] is cast. The channel is roughed into the wax and these tools are used to refine and finish the metal after casting so the stones can be set. You could carve the whole channel in the metal. You may want to plan your roughing in and then your finishing cut. You can also file or engrave the channel in. The thickness of your existing ring shank will be a consideration as well. Obviously, accuracy and patience will be the key in this operation. There is more description in the book 'The Complete Metalsmith [R&T # 62-010].
Hope this helps