Try pulling the cable tight, using a small vice grip clamp the cable so it cant slip, then tighten. What are you doing just adjusting your breaks? If so it just takes a little practice, but all you need is the proper size wrench. Look up how to tighten breaks. I think doing derailers is trickier, I usally pull it a little tighter than I would actually want, but when its all tight its right where I want. It is truly just all in the feel and that just takes practice to gain.