Place tomatoes in food processor or blender and pulse until liquid.
Place a cheesecloth over a cup and pour tomato mixture over cheesecloth, collecting the liquids in the cup. To attach the cheesecloth to the cup I like to use a rubber band. You can also use a fine mesh strainer. To speed up the process, press tomato liquid down with a spoon. Remove cheesecloth and discard tomato pieces on top.
Add celery salt into cup and stir. If serving chilled, add a couple of ice cubes to cup.