step 1
You have a set of data
Find the mean
the mean is equal to the sum of all values of the data set divided into the number of values
step 2
for each number of the data set, subtract the Mean and square the result
step 3
work out the mean of those squared differences
this value is called the "Variance"
step 4
Take the square root of the Variance
This value is called the standard deviation