Round each number in the question to only have one non-zero digit. For example, 64 becomes 60, and 1,890 becomes 2,000. Then do the calculation required.
Each number in the problem should be rounded to a single non-zero digit. If the second digit is five or greater, then add one to the first digit; otherwise, retain the first digit as it is. Set all the other digits to zero, so as to retain the place value. For instance, 27,656 becomes 30,000, 43 becomes 40, and 2 remains 2. Then perform the calculation required. This method of estimation can be used for any type of calculation, and the result is an estimate of the result of the full calculation.
For instance, if you want to add 456 to 882, first round the two numbers in the problem: 456 becomes 500, and 882 becomes 900. Now add 500 to 900 to get 1,400. This is an estimate of the answer. An example of division would be 4,310 divided by 521, which becomes 4,000 divided by 500 for an answer of 8.
This strategy does not always give very accurate estimates of an answer, but the calculations required are much easier than the calculations required for a more accurate answer.