If you wish to create custom HR fields, you must have HR+ activated. If not done yet, you can activate it under your Billing tab.
2) Creating a field
Once you are under Settings -> Human resources -> People Data you will have 3 sub sections:
Health and safety
In each of those sections, you can Add a field
Once you click on the button, you will be prompted to chose the type of field you wish to create. Then... Follow the steps!
A good thing to know is that on the left side of the creation panel, you will have a preview of the field in creations. You'll be able to see what the field will look like in the employees profiles!
3) Finding the field
Once created, your HR field will be found under the employee profile in the chosen section.