I need help creating masks for icons

Hi

I'm new to Figma, and I'd like to learn how to create masks for icons. What you see in the image are several icons from this design file. Each icon comes with a mask, and the mask follows the shape of the icon, It also has an alpha color, (semi-transparent pink) 

Please, I need step-by-step instructions to learn how to create masks that follow the shape of the icon. I'm a noob at Figma so I need your help. 

Thank you in advance.