Start with the wedding theme. Color, special flavor from your cake, season of the year (fall: apples, Spring: strawberries etc) all can give you an idea for the theme.

The base spirit is next. Pick something that you know your guests will like and that is affordable.

Next either hire a mixologist or take an existing drink and start to change one ingredient at a time. Add a new flavor, add or decrease the sweetener, add a different garnish, experiment.
Hold a taste testing party with your bridal party to not only create a memory but elicit their advice.

Make sure it can be made in sizable quantities. And make sure your venue and bartender have the necessary ingredients for the drink.

Having a signature drink can allow for a memorable drink and create a fun atmosphere!