Login to your club control panel and click the 'Membership' tab from the left-hand menu.
Within the sub-menu, click 'Fields'.
Create a new membership field or edit an existing membership field.
Towards the top of the page you'll be ask which roles you'd like the field to appear against, as seen below.
Further down the page, when entering field information you'll be presented with the option to make the field mandatory.
Click the 'Save' button at the bottom right of your screen.