Good, lengthy article on collecting and storing rainwater.
Several methods are show and include some video examples.
http://www.skilledsurvival.com/rainwater-collection/DO follow the embedded link about the legality of rain collection. This may be problem in your area, so do your research to stay under the rain police radar... This link is from November 2016 and you will need to check for ore current "regulation".
http://www.ncsl.org/research/environment-and-natural-resources/rainwater-harvesting.aspxIf you are in a bug-out situation, having a water resistant sheet of anything, a bedsheet, plastic tarp, shower curtain, etc., may be the difference between life and death...
If you have the option on your property, collecting rainwater for agricultural use is simply smarter.
DO NOTE the warning about roof drain water which NEEDS to be decontaminated for consumption...