Link to water harvesting expert Brad Lancaster's website. Lots of very helpful information including catchment and cistern sizing calculators: http://www.harvestingrainwater.com/rainwater-harvesting-inforesources/
Detailed "how-to" guide from NC State Cooperative Extension: http://wrrc.arizona.edu/sites/wrrc.arizona.edu/files/NC%20State_Urban%20waterways%20residential%20RWH%20guide.pdf