Open up the Triggers Interface
Let's review what the setup looks like for an automated trigger that will email the guest to request a remaining balance payment at 30 days prior to arrival.
The two conditions that would be required are:
Invoice Status, equal to, confirmed
Arrival date will be, equal to, 30 days
Property or Channel Variables
It might be desirable to limit the trigger to certain properties (default = all properties) or to exclude certain channels (default = all channels). For example, Airbnb collects payment from the guest. It would confuse the guest to send a payment request to an Airbnb guest (not to mention I don't think it would actually reach the guest as Airbnb would block it).
Setting the trigger to fire only once per reservation is advisable as well. Last minute reservations by default will require 100% payment at the time of booking.
Action and Email Body
Choose the action method = email only for this trigger
Choose who to send the email to = guest
Configure the text of the email.
Insert the required tags for the link to request the payment and (optional) a note about the amount of the payment due.
SAVE the trigger
Note if you send a test trigger to yourself, the payment request link will not be populated.