It is possible you will need to shoot slightly above her eye level for slimming and if you are a beginner at posing – you would probably have to shoot from above as it is safer to get the shot when you want to slim somebody and eliminate a double-chin or no chin.
Just be aware that shooting down on a person is an old-fashioned approach (which is fine when you want a particular old-fashioned look and feel) and may not feel modern any more. Of course there are exceptions, depending on the pose and effect you want to achieve.
What most important is to learn how to pose properly by stretching the neck, pulling the chin out, leaning forward with one shoulder, dropping that shoulder down, putting the weight on the back foot, pushing the back hip further away from camera.
When you are shooting slightly above the eyes and have them push their front shoulder down and chin forward - not only it eliminates a double-chin situation, but also make their eyes look bigger.
Wide angle lens, used skilfully, can help with slimming people down.
TIP. I often shoot from a very low position angled up with very wide lens in portrait mode, making sure the person is placed in the middle of the frame – that slims them right down and adds height.
Make sure it is only for full body shots and definitely make sure the person is leaning towards your camera slightly otherwise their head can appear too small, their chin can become too wide and eyes too small.