Adding: if you are adding a decimal with a whole number, put whatever the whole number is before the decimal to get the answer.
Example: 14 + 0.6 = 14.6

Subtracting: if you are subtracting a decimal from a whole number, take each place value away one at a time to get the answer.
Example: 10 - 0.546 = 9.454
10 - 0.5 = 9.5
9.5 - 0.04 = 9.46
9.46 - 0.006 = 9.454

Multiplying: if you are multiplying a decimal by a whole number, multiply the whole number by each place value. Then, add the numbers together to get the final answer.
Example: 0.324 x 3 = 0.972
3 x 0.3 = 0.9     3 x 0.02 = 0.06     3 x 0.004 = 0.012
0.9 + 0.06 = 0.96     0.96 + 0.012 = 0.972

Dividing: if you are dividing a decimal by a whole number by each place value one at a time. Then, add up the numbers to get the final answer.
Example: 0.14 ÷ 3 = 0.04666...(repeating)
0.1 ÷ 3 = 0.0333...(repeating)     0.04 ÷ 3 = 0.01333...(repeating)
0.0333...(repeating) + 0.01333...(repeating) = 0.04666...(repeating)