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Before you begin making the shake itself, get your serving glass ready by placing it in the refrigerator or freezer until it feels cold.  Keep the glass in the refrigerator for 30 to 40 minutes. Keep the glass in the freezer for about 10 to 15 minutes. Note that cups made of glass will get colder at a faster rate than those made from plastic. Make sure the glass you choose will be tall enough to fit the entire milkshake. Once your glass has been chilled, pour in the cold milk. Any dairy milk will work for this recipe: whole, 2 percent, 1 percent, and fat-free. You could also try soymilk or almond milk for a dairy-free alternative, but these options may slightly alter the final texture of the milk shake. Add the pistachio instant pudding to the milk in your glass and stir, continuously and rapidly, for 2 to 3 minutes.  The pudding mix must be completely dissolved. As it dissolves, it should start to thicken up the milk, but the effect may not be noticeable at first. The shake gets its flavor and color from the pudding mix. You can enjoy the pistachio shamrock shake as is, but if you want a thicker shake, you should let the drink sit in the refrigerator for 30 to 60 minutes. As it sets, the shake will thicken a great deal more. To speed the process up, you could also try sticking the shake in the freezer for about 10 to 15 minutes. You do not want the shake frozen solid. You only want it to get cold enough to thicken up a little. Serve immediately and top with a generous portion of whipped cream, if desired.
Chill your serving glass. Pour the milk into your glass. Stir in the dry pudding mix. Chill in the refrigerator for 30 minutes. Garnish with whipped cream, if desired.