NOTE: this activity has not been tested with a class of students. Please contact me with suggestions if you try it with students.
Cut the top off the 2L bottle, where the neck widens to the main body.
Tape a ruler on the inside of the bottle, the numbers facing outwards, with zero at the base of where the sides are parallel.
Add clean rocks (to weigh it down) then place in a shady location (to minimize water evaporation) with open sky above (no nearby wall or overhanging branches). Insert the top of the bottle upside down, and clip together.
Make sure the rain gauge is sitting level, then fill with water until the water line is at the zero on the ruler.
Hourly (if there is a lot of rain) or daily/weekly, read off how much rain has fallen. After each reading, either take apart and reset to the water line at zero on the ruler, or record the difference in mm of rain from the last reading.
Tabulate and graph the readings.
Optional set up to get more accurate readings:
Attach a narrow tube to the mouth of the bottle, so that the depth of water will change more dramatically for smaller readings.
Before adding the tube to the system, calculate the scale to add to the tube: calculate the relative surface area of the circles at the top of the 2L drink bottle and the small tube. This will be the relative difference between the spacing on the two scales (e.g. if the 2L bottle circle is 4 times larger, the scale on the small tube will be 4 times more spaced out). Calculate and write a new scale on electrical tape and tape to the side of the narrow tube, before inserting on the mouth of the bottle. Then clip the rain gauge together, as before.
To give a sense of what readings to expect, light rain is only 1mm an hour or less. To read 5mm in an hour it needs to be steady solid rain.
Note that despite the placement of the rain gauge, there will be some evaporation, so readings will not be as accurate as a professional rain gauge. A professional rain gauge style is "tipping bucket" - the rain fills up a little bucket, which is dumped and counted as it becomes full, hence there is way less sitting water to evaporate.