I'm studying stat from Andy Field's "DISCOVERING STATISTICS USING SPSS" 3rd ed.

In chapter 5 (Exploring assumptions) page 138 the writer said "To transform any

score to a z-score you simply subtract the mean of the distribution (in this case zero) and then divide by the standard deviation of the distribution (in this case we use the standard error). Skewness and kurtosis are converted to z-scores in exactly this way."

My question is : Why the mean is zero?

As far as I understand, the mean will be zero after converting a data to z score, not before conversion.

Can someone please help me to understand how to find the z score of skewness and kurtosis?

Thanks in advance.