Shooting into natural light
Well, apart from it being a bad idea, generally, to shoot the SHADOW SIDE of the subject against a vastly stronger light, which is what you are doing....
...you need to INCREASE the exposure with whichever manual override controls the camera provides, to lighten the parts that are too dark.
However, this is only a partial solution. When you do this the light parts of the picture, the windows, will get even lighter.... with the result that they burn out and flare across the shadow details and grey them out so that they look bad...
....become lacking in colour and contrast, not just lighter.
To avoid this, the correct thing to do is put more light, much more, into the shadow side of the subject, to balance the lighting and produce a brightness range narrow enough that the camera has half a chance of recording it.
Of course, that additional lighting itself requires considerable skill and expertise to place correctly, in order to flatter the models and their clothes. This skill requires a deal of natural talent, combined with months or years of practise. As it is, without a change of location, you would be learning the hard way with those windows messing everything up.
A skilled photographer would also use a better camera, too, but that is the least of your problems.
Conclusion: You'll probably get better pictures if you simply took your models outside and shot them by natural daylight.... if possible, on an overcast day. If the weather is not overcast, arrange for the direction of light to fall ACROSS the subject, instead of from behind, and have someone hold a soft silver or white reflector to bounce back some light into the shadow side of the subject.
Errr.... Does this help?
PS. For more specific advice, please post a sample of the type of results you are currently getting.
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