When you need to add staff custom fields you can navigate to the settings on the menu bar and find the staff on top.
Menu Bar >> Settings >> Staff
Click on Staff Fields.
When you enter the Staff fields section you will find some default categories and fields. You can use it as it is or customize them according to your school by editing and adding fields and categories.
To add new category, click '+Add New Category'.
When you add a new category you will get a popup page where you will need to add details like:
and click Submit.
After you have added a category you can start adding new custom fields in it.
Click '+Add New Field'
In this area add:
Click Submit to save.
You can repeat the same steps to create more fields.