It really is as simple as screwing in a light bulb with one of these
kits. If you have a fixture where the actual light socket comes out when you remove your old trim, you may have a little more trouble with the conversion. You may have to leave the old trim ring installed and hope that the little cover plate that comes with the pendant fits over it. It might be a trial and error type thing.
Make sure you measure the light and get a kit that will cover the old hole. If the light isn't an air tight type fixture it might serve you well to get a small piece of insulation and stick it up in there when you put the pendant in.