Find the centimeter marks on the ruler
Look for the smaller lines between each centimeter mark
Count each small line as 1 millimeter
Remember that 10 millimeters equal 1 centimeter
Read the number at the nearest whole centimeter mark
Count the millimeter lines past that centimeter mark
Add the millimeters to the centimeter reading
Use the longest small lines to identify half-centimeter marks
Use the shortest lines to identify individual millimeters