Fill a high ball glass (or a hurricane glass if you like it fancy) with fresh ice and pour in 2 ounces of whatever good quality vodka you have(I like Russian Standard or Absolut but any will do) and 1 ounce of blue curacao.
Top with lemonade and stir. I like to use sparkling lemonade or San Pellegrino, so I usually don't shake this cocktail, but if you're using regular lemonade feel free to toss it all in a shaker to mix it up more quickly. Garnish with a cherry and a slice of orange and you're on your way to that island paradise you always dream of. Enjoy your blue lagoon!
There are also quite a few variations of this recipe, I've listed a few of them on my personal website here - http://www.homebartendingguide.com/the-blue-lagoon-cocktail-plus-some-great-variations/