ok some things you will need:
it requires 4 ounces of gin and some of the best to use is Tanqueray. 1 ounce of Dry vermouth and for that i prefer Noilly Prat. 2 to 3 olives and a teaspoon or so of olive juice.
Start by putting ice in your martini glass to chill it down and set it aside. then put ice into your shaker ( or if you dont have one just use 2 cups that fit together just make sure there is a seal). Put the ounce of dry vermouth into the shaker followed by the gin and olive juice. cap it and shake until the bottom of the shaker becomes frosty. then pour the ice out of the martini glass and strain the cocktail into the glass. garnish and immediately serve. if you want a vodkatini then substitute the gin for vodka (try absolut, Smirnoff, or Grey Goose). if you want a Gibson then scrap the olive juice and olives and use a cocktail onion. and thats all there is to it. be careful these things are strong!