Open your iPhone's settings menu, type in your passcode, and press "Set Up Face ID."
You may also want to try turning off the "Require Attention for Face ID" option. I found that this improved my chances of getting Face ID to work while wearing my mask.
Keep in mind, though, that disabling this setting will make Face ID slightly less secure, as it removes the layer of authentication that requires your eyes to be open and looking at the screen to unlock your phone. I'd recommend turning this back on when you're no longer wearing your mask.