Log on via google chrome/Firefox.
Go to profile.
Click 'Manage listing'
Click on a recent review from a guest in the notifications part (I.e. john has left you a review!)
In the bottom right there is a place to click that says 'leave response'
The ability to leave a response is limited in time.
Just say thankyou. Welcome back. If its a bad review just suggest it was a failure for both of you to communicate.
I've got one guy who: left a puddle of urine in the bathroom, was up at 4am cooking sausages, made racist remarks, his room was filthy with stench, and half way through I went in and vacuumed, mopped, made the bed, hung up his clothes.
He was GROSS.
He invited a guest around after the guest cut off time. So I kicked them out. Abruptly.
He left a bad review, but I didn't respond. I didn't even leave him a review.
It isn't worth it.
The urine was pretty bad.