Look at the metric ruler
First you need to get a metric ruler and understand it. You will see lines which are big and many lines which are small. The smaller ones indicate millimeters while the bigger markings show centimeters. There may be other lines in the same series and also on the other side of the ruler. Do not worry about those as they are used to calculate or measure the distance between two points with a different scale. We are interested in measuring in centimeters, so you can disregard any other markings for now.
Remember that measurements are taken in decimal form.
Take measurements of the object
Next you need to use the ruler for measuring the object. In order to calculate the length of the object, you need to place the ruler besides it. Find the 0 ruler mark and place one end of the object in front of this. Be sure to hold the ruler and not move it around while you are trying to measure something. If you keep moving it then the measurement will not be accurate and you might want to start over again. Take your time while measuring so that you get an accurate number.
Count the centimeters
After placing zero point of ruler besides the end of object, measure the centimeters. Then count the number of centimeter lines which the object covers e.g. your object may be 7 centimeter long. Remember to double check when counting the centimeter lines. Use your finger or a pencil to make sure that you are counting the lines properly.
Determine the number of millimeter lines
In the end, you need to count the number of millimeter lines. This step is not necessary of your object is exactly 7 centimeter.
If the object you want to measure lays 6 millimeter lines beyond the 7 centimeter mark then the length of your object is 7.6 cm.