Summarize:

You can use the base of a cocktail shaker, a highball glass, or any other clear glass. You only need the glass to mix the juice. It is not the glass you will be drinking out of. A bar spoon is a special utensil with a long shaft, specially created for stirring cocktails.  Hold the spoon near the top of the twisted section with your thumb and first two fingers. Sink the spoon into your glass, near the side but not touching it. Twirl the shaft, moving both back and forth and up and down. Continue stirring for about 30 seconds. Chill the glass beforehand by keeping it in the freezer for 5 to 10 minutes, or by storing it in the refrigerator for 30 minutes. Go slowly and carefully to avoid spilling any of the drink. Do not shake or stir the soda water, since doing so may cause it to go flat. Instead, allow the liquids to combine lightly and naturally. Soda water is another great way to water down the bitterness of an extremely sour juice.
In a medium glass, combine the gin and lime juice. Mix the juice and gin using a bar spoon. Fill a chilled highball glass 1/2 to 3/4 full with ice. Pour the contents of the first glass into the second. Add the soda water to the drink. Add garnish and serve! Finished.