To write a review you'll need to create a Yelp user account if you don't already have one.
Once you're logged in, you can locate the business you're trying to review by using the search bar located at the top of any page on the website or from the search button on the app. Enter what you're looking for in the first window (either a specific business or a category), and the location in the second. There are also search filters you can use to narrow your results.
Once you've located the business page, it's simple: click or tap Write a Review and you'll be led through the remaining steps to finish.
Please note that if you're unable to locate the business, you may need to add it to Yelp.