This is a recipe typical from San Sebastian, on the Northern coast of Spain. It's usually served in glasses of half-a-liter capacity, I say this just so that you'll be able to manage the proportions better.

Place about six pieces of ice in the glass, then pour vodka until the glass is full to a quarter of its capacity. Pour the juice of one lemon, fill what's left of the glass with Cola (usually Coke). Rub the top of the glass with the lemon peel, then toss it to the glass.

You've got the mixture right if the lemon juice makes the drink taste a little sour.