Any time you shake or stir a cocktail, you need to strain out the ice. There are a few types of strainer used for this, but they all serve to hold back the ice while allowing the drink to pour through.
If you use a three-piece cocktail shaker, it has a strainer built into the lid. Remove the small cap, revealing the strainer holes in the lid and pour the drink through.
A Hawthorne strainer is a flat metal strainer with a circular spring around the edge. This fits over the top of your shaker tin with the spring along the inside edge. Hold it in place with your index finger while you pour the cocktail through.
Finally, Julep strainer is like a large flat spoon with several holes in it. It’s often used when pouring from a Boston shaker or mixing glass. Place it inside the shaker atop the ice. Hold it in place with your finger while you tip the shaker and pour through it.
When you prepare a drink with small bits (such as torn mint leaves), the methods above won’t strain them all out. For these drinks, you should place a regular strainer on your shaker and hold a fine mesh strainer above the glass. Pour the drink through both strainers. This is called double straining.
Any drink with egg in it should also be double strained. This will catch any bits of the egg don’t fully incorporate during shaking. Some bartenders will double strain most shaken drinks, to filter out any small shards of ice and prevent them from further diluting the drink in the glass.