When writing a wedding congratulations card, the first step is to decide whether you want the card to be formal, casual, serious, funny or religious. An example of a formal message is "Congratulations on your new journey! May your years ahead be filled with joy."
"Best wishes on your special day," is another formal message, while "Welcome to the family!" imparts a more casual tone. "May God bless you both as you begin this new adventure," obviously has religious overtones.
If you write a message that's intended to be funny, make sure the joke isn't offensive and doesn't include inappropriate topics such as cheating or divorce. Wedding congratulations cards don't have to be lengthy as long as they are sincere.