I tried this feature when it came out a couple of weeks ago. I thought I would have to create a scheduled message for each and every guest, but once you set the parameters, it's applied to every guest booked for that listing. The only thing I'll have to remember is following up with any personalizations necessary. For now, though, it's great to open my inbox and found that I've just sent a message to a guest coming in ten days with all the info he or she needs.
If you haven't set them up, it's pretty easy. Go to Inbox, then choose the three horizontal lines in the upper left next to "All Messages." From there, go down to "Scheduled Messages." You'll have the option to add a new message or to edit the ones you've created.
When you create one, you can give it a name. My messages so far are all the information for a guest's upcoming trip, so I named one Cabin - Upcoming Trip. The shortcodes are useful. Type "Dear," then choose a shortcode for the guest's first name. Type the message you want to send or paste it in from previous saved messages. (You might need to have two tabs or windows open here, so you can easily access all your saved messages.) Once you're done creating the text, scroll down and choose which listing it will apply to. Then schedule it. The choices are "Booking Confirmed," "Check-in," and "Checkout." In this example, I'm sending check-in instructions, driving directions, and the house manual 10 days before check-in. After choosing "check-in," you'll be given date and time options. The dates range from 14 days before to 14 days after, and the time is of course any time of the day or night. You can also create messages that are sent upon booking confirmations and messages that are tied to the checkout date.
If anyone would like screenshots of the process I'll be happy to provide them.
@Ann72 I have been using this feature and am happy not to have to pay Host Tools anymore—I have found one bug to be aware of: a reservation was extended with an earlier check-in date, and the scheduled pre-check-in message vanished without being sent, maybe because the change put the time it was to be sent in the past.
Here are some screen shots for clarification!
When you're in the Inbox, go to the three horizontal lines to the left of "All Messages":
Click on those lines, and you'll see this list drop down on the left, with "Scheduled Messages" at the bottom:
Click on that, and you'll see +New Message over to the right:
Click on that, and you'll get a box where you'll enter all the info. Here it is in two shots. Don't forget to scroll down: