Take a thick, heavy glass out of the chiller/fridge, or just chill it with crushed ice & soda water for 2 or more minutes (a martini glass also works well)
Roll the lime on the counter, cut it in two and squeeze the juice out of it if necessary
Pour all ingredients in a shaker, close it tightly and shake it hard for a short time
If you chilled the glass with crushed ice empty it now; if your guest prefer a salted rim glass then pour some coarse sea or kosher salt in a dish larger than the glass diameter, and just touch the salt with the glass rim while holding the glass upside down (this assumes the glass is wet from chilling, else wet the glass rim with a lime wedge like this.
Double strain the Margarita from shaker into the glass
Garnish the Margarita with a twisted lime zest or even a lime wedge and serve right away!