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1. Stand in shade, not direct sunlight. This is to avoid hard shadows.
2. Light her face so that she is brighter than the background.
3. Use a softbox or a large reflector when you light her. Again so that there's no hard shadows on her face.
4. Pay attention to framing. Pic related.
Those are the main things I noticed newbies doing wrong.
And of course you should use a tripod, low ISO and all that jazz.