Step 1: here we have to reduce each number in the birth date (month/day/year) to a Master number, which is simply a single digit number. 
Step 2: we add-up each of the digits (or Master numbers) we got in the previous step. The total number we get must be again reduced to a single-digit one.

For Example:

March 30, 1989.
March is the 3rd month in a year. Let’s make it a single-digit: 3 (if your birth month is October you do like the following: 1 + 0 = 1)
Birth day is 30. Sum up the numbers in your birth day: 3 + 0 = 3
The year of birth is 1989. We have to reduce it as following: 1989 -> 1 + 9 + 8 + 9 = 27. 2 + 7 = 9. The year 1989 is now reduced to 9.

Next step: we now have to add the single digits we got from previous calculations.

month is 3, day - 3, year - 9. Total: 3 + 3 + 9 = 15, 1 + 5 = 6

Above we have determined that the Life Path number of a given birth date is 6.

There is another example: October 3, 1981
October is the 10th month in the calendar. The 10 is a so-called ‘Master number’ and is not reduced to any other number/digit. 
The 3rd day is a Master number too and kept as is: 3. 
The year 1981 is reduced to a single digit: 1 + 9 + 8 + 1 = 19, 1 + 9 = 10, 1 + 0 = 1.

The results we got:

So Life Path number in this case is 5. (if you have double-digit number in ‘total’ field – reduce it to a 1-digit).