Look at reference photos and if all else fails, wrap a towel around yourself and look in a mirror. Towels are incredibly useful if you need to see how a heavier fabric is going to drape. For a light fabric use sheets folded in half.
Use reference. Look up photos of women sitting in skirts. Put a tablecloth over a pillow placed on a chair and see how it acts. Practice drawing drapery from life to learn how gravity affects clothes. Also take a look at this: [link]
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I'm trying to draw her sitting down, but I 'm having trouble trying to draw her long skirt as she sits.
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I'm not that good drawing women sitting down in long dresses, so can anyone lend me some tips on drawing this?