Firstly remember to wear a dark coloured frock or you will get suspicious stains on you (probably not for the first time
On the standard jack there is a slit in the top piece that touches the underside on of the car. This slit should be placed where the cill meets the body of the car (the cill slots in nicely to the jack) about 10 inches back from the front of the cill, if you havent got a measuring tape then improvise with a bit of your anatomy..
(I tryed to take a photo of the bit but its not to clear...)then wind up the jack!
Would be an idea to get a trolley jack and an axle stand as you shouldnt have any part of your body under the car without properly supporting it first..(see below)