Summarize:

Use a pair of scissors to carefully cut the top of the bottle off. Cut just beneath where the bottle begins to narrow. A plastic soda bottle works best. Be sure to remove the paper label completely. Parental supervision may be necessary for younger children when cutting the bottle. Pour in the pebbles in the bottom of the bottle. Soda bottles are never flat. Pour in the pebbles to even out the bottom and to keep the gauge from tipping over if it’s windy or the downpour is extremely heavy. Remove the cap from the top of the bottle and turn it upside down. Place it upside down on the bottom part of the bottle so that the top is pointing downward. Secure the funnel in place by lining up the cut edges and taping the two halves together. Make sure that the top half is firmly held in place, and that there aren't any gaps between the two pieces of your gauge. Cut a long piece of duct tape and place it on the side of the gauge so that it creates a straight vertical line from the bottom of the bottle to the top. Take a marker and a straightedge to draw a horizontal line just above the pebbles. This is the bottom of the gauge. Use duct tape for it’s strong adhesive properties. Masking tape, or other forms of tape, may come loose when soaking wet. Take a ruler and place it against the tape so that the 0 lines up with the bottom line you have marked off. Use a marker to mark off every half-centimeter or half-inch along the tape until you get to the top. Label the centimetres or inches from the top to the bottom. You want to be sure that your measurements will be easy to read throughout the duration of the experiment.  Every half interval does not need to be labeled. Label every other mark by beginning with the second mark as 1 inch or cm. The marks in-between each label are a half measurement. Make sure to wait until your marker dries on the tape before placing your gauge in the rain. Do not use washable markers or apply markings while in the rain. If you have to reapply new tape or new markings midway through the experiment, your results may be considered inaccurate. You may use any unit of measurement your experiment calls for. You can mark off every quarter-inch, eighth-inch, or centimeter. Set your gauge on a level surface. Make sure it isn’t obstructed by any branches or in the way of people walking by. Pour some water in the bottom until it reaches the 0 mark and then you’re ready to collect.  You may also use coloured jelly instead of water to give yourself a reference point of where to start measuring. Use jelly or oil rather than another liquid as those may dissolve and mix into the water, ruining your measurements. These bottles are not evenly shaped on the bottom so you must account for this by establishing your starting point. Make sure that the gauge is in a protected area. You want to ensure that there’s no disturbance from wind, debris, or anything else that could block or prevent rain from entering, like a branch or powerline. Check the forecast for rain. Check your gauge exactly 24 hours later to see how high the water has risen. Now you know how much rain has fallen. Check how closely your measurements compare to official ones by checking the news or online for the rainfall measurements of your area for the day. You can continue measuring for 7 - 14 days or until your curiosity has stopped. If this is an experiment assigned by your teacher, make sure you follow every perimeter and take measurements until the experiment tells you to stop. Try to take your measurements at roughly the same time so that you get a 24-hour reading. Then, dump the rain out between each measurement to start a new measurement for the next day.
Cut the top off of the bottle. Place pebbles in the bottom. Turn the top of the bottle into a funnel. Create your measuring line. Mark off half-centimeter or half-inch intervals. Place the gauge at an optimal location. Pay attention to the weather. Repeat your measurements.