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Near your head. Never on the opposite side of the car. That will cause the car to pivot towards you when the jack fails.
I always use two + the jack. Some people use the factory emergency jack as a jack stand, which works fine.
If I work on the front, I jack the car up at the factory location, then put a jack stand between the jack and the wheel. Another near the subframe mounts.
In the back, one between the jack and wheel, another near the swaybar mount.