Measuring Rain
You can collect rainfall in anything where you can measure the depth in millimetres. But if you buy an open rain gauge, some of the rainfall can evaporate before it is measured. So it is best to buy a purpose built one if possible and many different types are available cheaply. You can make your own too. In remote locations you can't always check a rain gauge every day so bucket-siphon rain gauges take measurements and empty themselves daily, measuring the results on a paper roll.

How do I use a Rain Gauge
Place them in open space, more than 5 or 10 metres from buildings and trees, The measuring scale takes into account the area of the gauge so you can calculate a rainfall figure per unit area. Take a measure every day at the same time.