This website explains how to create a rainwater collection and explain all of needed steps in that process. 

1. "It all starts with your ROOF SURFACE.  Nearly all roof surfaces are fine for rainwater harvesting.

2. Your gutters should have some sort of GUTTER PROTECTION SCREENING in order to keep large debris from entering the gutters.

3. You can collect rainwater from any type and shape of GUTTER.  You do not need a special gutter for harvesting rainwater.

4. An additional filtration opportunity is with the installation of a RAIN HEAD (downspout filter) that allows for a bit of self-cleaning filtration options.

5. A FIRST-FLUSH DIVERTER helps to prevent the first flush of contaminated rainwater from entering the tank.

6. Another rainwater filtration opportunity is with a TANK SCREEN that is installed on the tank entry point.  It also helps to keep mosquitoes and pests out.

7. RAINWATER TANKS come in all sorts of sizes and materials.  Click to learn more about all the various types of rainwater harvesting tanks available on the market.

8. Install an INSECT PROOF FLAP VALVE on the end of the overflow pipe to keep mosquitoes and pests out.

9. An AUTO-FILL SYSTEM can be installed to keep a minimum amount of water in the tank at all times.  This is very important for rainwater tanks that are connected to automatic irrigation systems in order to prevent the pump from running dry.

10. Select a PUMP SYSTEM to provide pressurized rainwater to distribute the rainwater easier or to connect to an inground irrigation system.

11. Install an IRRIGATION FILTER inline after the pump in order to catch any large debris that may have gotten through the pump.

12. A WATER LEVEL INDICATOR can be helpful with monitoring the water usage from the tank.  Those are available in simple gauge tank options along with wireless digital options."

Research found at: https://www.watercache.com/education/rainwater-harvesting-101