How do you use front end estimation in math?

To use front end estimation, round the left-most digit of each number to be added, and replace every other digit with zeros. Repeat this process for any numbers following the decimal point. This method is a quick way to roughly calculate the sum or difference of a series of numbers.

For example, take 12 + 23. According to front end estimation, 12 needs to be rounded down to 10 and 23 should be rounded down to 20. The updated equation for the estimation is 10 + 20, giving an estimation of 30. This number is only slightly less than the actual answer of 35.