Posted on 12 April 18 at 07:36
First, make sure the game is on your TSA account. Then, go to the game's achievement page and at the top of this page, click on "Review".
If there are currently no reviews for the game, you should see "There are currently no user reviews for this game. Why not be the first to write one?" - click on the "write one" which will bring you to the text editor.
If there are reviews already, scroll to the bottom of the page and you should see "Think you can do better? Click here to add your own review for this game".