Type a (part of a) username in the address field. A spinner will show indicating the app is searching for usernames on Arena. Once usernames are found they will be shown in a list. Select the user you want to tip. The address for the user will be filled in the address field. Instead of searching for a username you can also directly paste an address. Make sure you paste the correct one.

Next type the amount of tokens you want to tip and click the "Send Tip!" button. This will open a Metamask compatible wallet where you can sign the transaction. After the transaction completes you can go check on Arena for a tip notification post by the @coqmon account.