First, take the blackberries and simple syrup and muddle them together in the cocktail shaker. That means you'll squish them until they're all mixed up.

Add the white rum and lime juice into the shaker.

Fill the shaker with ice, put the top on, and shake it like you're dancing for about 10 seconds.

Use the fine mesh strainer to pour the drink into your glass over new ice cubes. This keeps the seeds out.

You can throw a blackberry on top or a slice of lime for a fancy look.