You can leave a review after being a member for at least 15 days. Once that is fulfilled, you'll see a pop-up when you log into your service.
For the first 15 days you're using the service, you will not have the ability to leave a review, as we believe it's important to really familiarise yourself with the service before sharing your experience with others.
To do this proactively, you can click on "Tools" and select "Write a Review" from the list:
You can also leave a review by going to the URL for your author, which will look like this (with the author's name replaced by your author's name): https://seekingalpha.com/author/author-name/research/reviews/news.