Capturing rainwater can allow you to limit your water usage by using rainwater for things like watering the lawn or garden.

This means having working gutters that will drain as much of the water as possible to one point.

Figure out how large of one you will want by figuring out how much water you use outdoors on a regular basis and also being realistic about how much rain you are likely to get.

Attach your cistern to your water source you identified in step 1. You're going to need to filter this water for any large debris before it enters the cistern. You also need to figure out how you are going to get the water out. How you do this will likely depend on the type of cistern you have.

Use your stored water for as much of your outdoor water needs as possible.