Juice lemons into a small bowl. You can either use an electric or handheld juicer, a reamer, a stand mixer, or quarter them and squeeze the juice out by hand. Set aside.
In a cocktail shaker, muddle the mint leaves and sugar together. This releases the mint's flavor and combines it with the sweetness of the sugar.
Add the freshly squeezed lemon juice to the cocktail shaker. Shake or stir well to dissolve the sugar and mix the flavors.
Fill a tall glass or a mason jar with ice cubes.
Pour the mojito mixture into the glass then top with sparkling water or soda until it reaches the rim. Stir gently. Garnish with a slice of lemon and a sprig of mint.
Notes
By the pitcher: Pre-assemble the muddled mint, sugar, and lemon juice and store them in a container. When it's time to serve, simply combine the lemon mixture with sparkling water and ice.