STEP1:
From the Menu click on the RSVP Page tab:
Or from the Dashboard, Click on the RSVP Page button
The below screen will be displayed:
STEP2: Edit RSVP Page
Click on the Edit button 
on the right hand side of the page, the following screen will be seen:
Create a personal message that will be displayed on the RSVP page for your guests. Click on the Save button when you are done