This is a simple rain gauge made from a plastic bottle. My father tought me how to do it.
We always use it for deciding if and when we should water our fruit trees. (keep in mind that on a hot summer day 4 or 5 mm of water a day evaporated from the ground, so if it rained 50 mm, you don't need to water your trees for about 10/12 days).

Step 1: WHAT YOU NEED

MATERIALS:
_plastic bottle (conic or funnel shaped);
_strong tape;
_stick or cane.
TOOLS:
_cutter;
_permanent marker;
_scales or capacity measure for liquids;
_ruler.

Step 2: PREPARE BOTTLE

remove labels with warm water, and cut away the bottom of the bottle with cutter.

Step 3: MAKE MESUREMENT

_measure the open AREA of the bottle opening: r • r • 3.14 (measure the diameter, divided it in 2 and find the radius, then
      do radius by radius by 3.14);
_now, knowing that if it rain 1l of water on 1 square meter it will correspond at 1 mm of water, we need to find out how many ml in your bottle that mm of rain fall on the ground will correspond (x). We need to use this formula:

      1m²           bottle open area                                    1liter   • bottle open area
   --------   =    ---------------------               so:            x= ----------------------------------
     1 liter                    x                                                                 1m²

this is an example using the measurement of my bottle, my bottle diameter was 8.6 cm ( than 4.3 cm of radius)
4.3cm•4.3cm•3.14=58cm²( 58cm²=0.0058m²)

      1m²           0.0058m²                                            1 liter • 0.0058 m²
     --------- = ------------------                   so:           x= ---------------------------         so:       x=5.8 ml
      1 liter               x                                                             1m²

another way to find your x is: calculate the volume of 1 mm of water on the area of open surface of your water bottle, and use that number to make the measurements on your bottle:
r•r•3.14•0.1cm²=x    (than transform it: 1cm²=1ml)

Step 4: MARK NOTCH ON BOTTLE

now you have found x.
_x is the corresponding measure on your bottle of 1 mm of rain fallen on the ground, so we need to use it to mark the measurement on the bottle.
_because the number you will find is so small I will multiplied it by 5 times (so it will correspond to 5 mm of rain fallen on the ground).
_pour in the bottle the milliliters of water you just measured (5x) and mark it with permanent marker (write 5 mm). you can use a capacity measure for liquids or a scales (1l of water weigh one kilo).
_keep adding milliliters and marking until you mark the whole bottle.

example of my bottle:
_my x were 5.8 ml
_5.8 ml•5= 29 ml (corresponding at 5 mm of rain fallen on the ground)
_I signed the 5, 10, 15, 20,30,40,50,70,100 and 150 mm of rain on my bottle.

Step 5: MAKE THE PEDESTAL

place the stick on the bottle (like in the photo) and stick it with tape.

Step 6: DONE

you 're done! place the pluviometer (rain gauge) in your garden (stick the cane in the dirt), far from trees or roofs.
empty after every rain: you just need to unscrew the cap of the bottle.