The three-part shaker with the lid and the built in strainer—the Pottery Barn one you received as a gift—isn't necessary. What you need is a classic Boston shaker (Rosale shaker, $50, at Sur la Table). Just make sure you've got the hang of it before swinging it over your shoulder.
Fill glass with ingredients (alcohol first, mixers second).
Add ice until 3/4 full.
Rest the metal tumbler on top and with the palm of your hand, give it a generous tap to create a seal.
Grab the metal with one hand, the glass with the other, and shake until the metal tumbler is sufficiently cold (about 15 seconds).
Set it down (glass side down) and break the seal by thumping the point where the metal meets the glass.
Keeping the two pieces together, turn horizontal and position the metal tumbler's edge over your glass. Slightly pull the two rims apart, pouring the cocktail but retaining the ice.