Where are you taking it? If you take it to Wick & Greene or Karat patch, they will be able to do it with little to no instruction. Heck, W&G made my band to match my engagement ring with just a minor description of the Ering...There are other good jewelers here I know, those are just the only two I've had personal experience with.
Are you going to get rid of one band altogether, or are you combining the two? Does that make sense? I get that you're combining stones, I just don't get if you're having the bands merged together, so to speak.