I sent 3 automated messages:
1. When they make a booking to thank them and let them know that I will send checkin instructions 1 day before check in date.
2. Very detailed check in instructions 24 hours before check in.
3. Check out instructions and thank you note 24 hours before check out.
I do check with them after their arrival to see how everything is in the apartment. Ask if the check in was smooth, if everything is good in the apartment and again if they need recommendations for places to eat or see. I think guests really appreciate that follow up. As other folks mentioned here, it also covers me (to some extent) in case guest claim after they leave that the apartment was not to their standards or something didn't work. Usually they let me know quickly if something breaks.
After that email a day after they check in, I don't communicate so they can enjoy their time and not be bothered. I get 5 star rating for communication so I think it works 🙂