After you choose your card design you are taken to the DESIGN tab of your event. In the EDIT TEXT tab you can edit and design your text.
As you can see on the left side of the image below you can choose a font and edit color, spacing, line height etc. You can edit the text on the card by clicking on the text on the card image.
Here you can also personlize your card by clicking on the grey button "SHOW RECIPIENT'S NAME(S)". A text field for your recipient's name is added to your card. You can edit the font and color of this text field. The contents for the name are taken from your recipient's list. Adding the recipient's name costs 1 additional stamp per sending. Please see What features cost extra stamps? for more info.