Start by putting a Collins glass or a highball glass in the refrigerator to cool. A properly chilled serving glass adds to both the taste and visual appeal of a drink, so make sure you give it about 15-20 minutes in the refrigerator.
Now you can begin preparing your drink. Take a cocktail mixing glass (make sure it is absolutely clean, as any additional flavours could taint the cocktail), and fill about half of it with ice cubes. Cracked ice will also work fine for this purpose, but might water the drink down a bit.
Then, once the mixing glass is packed with the ice, proceed to pour the gin, lemon juice and sugar syrup into it. Use your stirring rod to swirl everything around until all the ingredients are completely mixed and the drink is thoroughly blended.
Now, take your chilled serving glass out of the refrigerator, hold a strainer over it (to catch any shards of ice, or lemon seeds), and pour your blended drink into the glass. Then, top it off with some club soda.
Finally, garnish the drink with a lemon slice, and if you wish, you can also add two to three ice cubes to the glass. Your Tom Collins cocktail is now ready – serve chilled.