There are a ton of threads here on Cold Water Extraction as well as a lot of resources online to help you out. Just have to search, man. But I'll do what I can since I'm here already haha.
I'm assuming you basically know how to do it but just want to know how to get better at it? To obtain the highest yield, use the pills that contain ASA (Aspirin) instead of Acetaminophen or Ibuprophen. They filter much better, and usually dissolve faster in my experience. With a little bit of practice you can get the yield up to somewhere around 95%, so you're really not losing much Codeine at all. Also, as a general tip, ALWAYS wet the filter before pouring in the solution to keep it from soaking up your codeine mixture.