There are many nicely designed rainwater harvesting systems in the market and most of them allow the users to customize the components for their intended use (indoor /outdoor / drinking etc.) but the more functions you want, the higher the cost will be. However, if you want to own a rainwater harvesting system for gardening only but do not want to spend too much of time and money, this DIY guide is suitable for you.
Image Source: Underground Health
The diagram is pretty much self-explanatory but if you need more precise instructions you can always ask the pros here.