Take an empty 2 litre plastic spring water bottle as found in all supermarkets. Cut off the top of this bottle about a quarter of the way down where it reaches a consistent diameter. Be careful to cut this smoothly.
Simply remove the bottle top and invert the upper part of the bottle into the bottom part.
It should fit snugly but to make sure it does not fall out use a few paper-clips to hold the two halves together
Place your rain gauge in a location where it will collect rain without being affected by buildings or trees etc.
To help it stand upright you might use a bucket of ballast (pea-shingle or sand or similar) which can be mounded up each time after taking your recording
Every day at the prescribed time visit the rain gauge and carefully remove it from its ballast. Take out the inverted top and carefully pour the rain collected into another container, preferably a measuring cylinder.
Remember that there is a small section at the end of the ruler that you should exclude from your measurements Also very important is the fact that these bottles are usually shaped at the bottom. You need a completely flat bottom to be able to measure depth accurately. One way of getting this would be to use some brightly coloured jelly mixture and let it set in the bottom of the bottle. Then you can measure the depth from the top of the jelly where the water starts. You can always eat the rest of the jelly!