Estimation with addition and subtraction: Round each number to the Highest COMMON place value. If both number go to the same place value, you can just round them to the highest place value. However, if one number goes to a higher place value, you have to round to the highest place value of the smaller number. ex: 123,456 + 78,012 Both numbers would be rounded to the ten-thousands place. 120,000 + 80,000.
Estimation with multiplication: Round each number to its own highest place value and keep single digits the same. (All the single digits!)
Estimation with division: Round the divisor first (but not if it is a single digit) and find a number that is compatible with the divisor. (pretend that the ~ is a division sign!) ex: 4,812 ~ 7 would change to 4,900 ~ 7 because the 49 works really well with the 7.
I hope that this helps a little bit! And remember not to round the single digits, all the single digits!
-Miss K :)