Pet fees must be setup and defined within all pet friendly properties. Setup instructions are here. Once that is done adding a pet fee to an invoice is easy.
Note: Pet fees can be added manually to an invoice as "one-time fees" by typing in the name of the fee and defining the fee amount. However doing it this way doesn't allow you to report on the amount of pet fees collected for any given period. It's better to define the fee and the add the fee as an "additional fee" rather than adding it as a "one-time fee".
Click the green "+" icon to add a new predefined fee
Choose "Pet Fee" (or whatever your called it when you set it up)
Click the Save Icon
Once saved you can add a quantity and assign a tax to it if applicable. See next step.
Add the quantity of pets, select any taxes that need to be charged to the fee and then adjust the amount per pet (if necessary), click Save and you are done!