How to review a game in Steam library?
Solution 1:
If you scroll down on the game page in your library, there is a button that lets you post a review:
Alternatively, if you have played a game for some time but have yet to write a review, there is a feedback panel that leads to a review prompt page:
Which leads to:
Solution 2:
You can write a review for a game you own from the game's store page. You can reach the store page through the link under the install/play button on the game's entry in your library.