Don't lay under it with one jack. That would be stupid.
At a minimun, have one jack and one jack stand. Shake the car to make sure is steady.
DO NOT DO ON AN INCLINE!
Use ebrake if you have one and block rear tires.
Yes, I suppose I should have added the requisite safety disclaimer, rather than simply answering the question asked and nothing more.
TWO jack stands, never ever trust a hydraulic jack!
Good places include under the lower control arm pivots,
or under the lower shock mounts (suspension will compress slightly)
Most other places under there have too much of a curve for me to trust.
I don't like the frame rails cause they are too high up there and I don't trust long jackstands.
And while we're at it, if you put your jack pad right in the center of the crossmember, the crossmemeber will bend or even cave in.
I like to catch the end of that "tongue". Not too far up the tongue though, or the jack will slip and get spit out. I lost an alluminum radiator to that once!!!
And yes, even with the lowest profile jack I've found, I have to lift my car by the bumper while kicking the jack under there!