You have to have bought or installed the app in order to post a review. On Google Play on your device, you should see "Your review" and stars to the right of it, just above "What's new." Tap on that, and you will have the option to leave a review.
1. the app should be installed on your device for you to review
2. open the app on play store and it will say "Rate and review" click on it to rate and review
Unfortunately you're unable to search by stars, but it does give you a few other options. It usually displays about 3 reviews. Scroll down and you will see a link for "All Reviews". Tap on it, then look to the left for a drop down menu that says "Most helpful first". That's usually the default. You can change it to Newest First or Highest Rating First.
I do wish they would give us the ability to view them by rankings, or at least add the Lowest Rating to the list.