Use the map search function to find the map you wish to review.
Follow the 'review this map' link that should appear. This link will only appear if you are logged in (and you aren't the author of the map.)
copy-paste the text of your review into the web form. If using HTML, check the appropriate box.
Submit. Your review will be saved on the system, and a copy emailed to the review editor, who may get in touch with you (especially if you're a first time reviewer.) When they have approved the review, it will appear to other visitors to MapHaven.