As suggested use the Park Tool or Sheldon Brown procedures; I prefer Park's explanation myself:
http://www.parktool.com/blog/repair-...nts-derailleur
http://www.parktool.com/blog/repair-...ur-adjustments
Whichever you use start right at the beginning and DO NOT skip any steps as each adjustment depends to some extent on the previous step being done correctly. It is important that at the beginning you set the shifters (whatever type you have) to have released all of the cable that they have taken up. It is also important that the shift cables move freely and easily. If any of the cables or housings are not in good condition, remedy that first. Pay special attention to the final loop of cable that enters the rear derailleur, it is often damaged or jammed with dirt or rust. Park and Sheldon also have instruction on cable and housing installation, a poor installation is often the cause of shifting problems down the road, so to speak. Also make sure that the shifters pull and release cable properly and smoothly. You can do this right, just take your time and be patient and methodical.