The goal is to achieve equal pay for equal work. Group your employees into sensible categories of comparable work. At INC, we used job descriptions and job titles. The AIA has some defined categories. Based on the size of our studio, we ended up with 3 groups. Keep it simple, too many groups will be too difficult to evaluate. List salaries for each employee. Within each group, simply calculate mean and median wage for each gender and calculate the gap. We check our metrics at least a couple times a year. Refer to UK detailed instructions if you like.