As with all cocktails there are different variations, so can't guarantee that your visitors won't find something to complain about! But the standard way to do it (that I use and tastes great) is:
* 2 parts vokda (if you can afford it Absolut or Finlandia taste the smoothest)
* 1 part Cointreau (a cheaper option is triple sec - another orange-flavoured liquer, but it's a bit rough on the tongue!)
* 1 part cranberry juice
* juice of half a lime (I buy those Jif bottles of lime juice - much less hassle and tastes fine)
I got a couple of cheap martini glasses from Ikea, which are also 'Cosmo' glasses. For proper authenticity, mix the drink in a shaker with ice cubes and strain the ice off before serving (or, if not too bothered, just pour the ingredients straight into the glass over ice cubes)
For the proper finish (if you can be bothered!), flame a piece of orange peel over the top - basically you have to squeeze the outer side of the peel over the drink and as the orange oil squirts out set fire to it with a cigarette lighter, leaving a smattering of oil over the top of the drink. Never done it myself but I've seen it done in cocktail bars - and it does make it taste more orangey.
Word of warning - despite it's pink colour, a Cosmo is not a knock-back-in-one light cocktail. The cranberry flavour is barely noticeable and it's basically a knock-yer-socks off vodka drink. For timid palettes more cranberry juice can be added - but this wouldn't be an authentic Cosmo.