One, thanks for wanting to write a review. Reviews are like the magically cocoa beans that keep this economy running.
Two, on to writing!
Go to your guest portal, or click "Guesting" from your drop down menu.
From here, you can choose the meal you wish to review by clicking "Guests" on the same line as the title
At the top of the guest list will be the host, and you can click "review" next to their name.
Below the host, you will see the guest list, and can review other guests that way.