To prepare your margarita glass
Coarsely grind some salt onto a plate or shallow bowl
Wipe the rim of the glass with lime juice (you can use a quarter of lime, or just use the shell of the lime you just squeezed for lime juice).
Dip the rim of the glass into the salt, rotating as you go, until it is fully coated.
To make the margarita
Measure the Tequila, Cointreau and lime juice into a cocktail shaker.
Slice the jalapeno pepper into discs and remove the seeds (or don't if you want it spicier). Add two or three discs to the shaker (I recommend starting with two slices).
Half fill the cocktail shaker with ice cubes. Tightly secure the lid and give it a good shake for at least 10 seconds.
Taste the margarita. Add a further disc of jalapeno if you want it spicier, and/or sugar syrup if you want it sweeter then shake again.
Fill your prepared margarita glass with ice cubes and use a cocktail strainer to strain the margarita over the ice.
Garnish with a slice of lime and a slice of jalapeno pepper.