First, head to the listing you want to import the review to by accessing its URL or via Host > Listings. Notice: your listing should be published and you should access it's live view to add your past reviews. Once you have reached the listing page, you will find the Reviews menu option, or just navigate down where you will see it. Click the 'Write Review' button and then start filling the available inputs:
Title: Title your review, it could be a short brief about the guest experience on the listing or anything you want related to his/her stay.
Review: Copy/paste the review that guest left on your listing,
Rating: Give a rating to the listing! Just click one of the stars that go from Bad to Excellent to summarize guest's experience on the listing.
Guest name: Enter the name of the guest.
Guest e-mail: Input the e-mail of your guest. This field is optional, and only you will be able to see it on the listing page.
Source: Paste the URL from the source of this review. You can simply copy/paste the link from your browser.
Dates of trip: Inform the dates of your trip, click check in or check out option and pick the date.
Once you have filled out the review form, you can hit the Post Review button. You'll see your review on your listing page shortly: