Compute the mean: (mu = frac{sum x_i}{n})
Compute the standard deviation (population): (sigma = sqrt{frac{sum (x_i-mu)^2}{n}})
Compute the standard deviation (sample): (s = sqrt{frac{sum (x_i-mu)^2}{n-1}})
Use the appropriate spread in the formula:
Population: (z = frac{x-mu}{sigma})
Sample: (z = frac{x-mu}{s})
Plug in the value (x), the mean (mu), and the standard deviation ((sigma) or (s))
Simplify to get the z-score value