If you look under the side of the car, you will see a long flange where the sides of the car meet the floor pan. There is some plastic trim that runs along most of it. However, at the front and the back there are a couple places where the plastic skirt opens up and leaves you with a 6inch-ish gap with access to the welded flange. In the middle of that gap there will be a beefier weld that has two little dimples about 2 inches apart. Your jack goes in that slot.
Seriously though, go buy yourself a cheap 2-ton hydraulic jack. You'll thank yourself later. And for safety, don't trust crawling under a car only supported by a jack.