My stylist blow-dried my hair and then flat-ironed it, since my natural hair was wavier than the extensions. With extensions, you can style your hair as usual; just make sure not to apply heat directly to the bonds.
The hair felt incredibly light and natural right away, and even with it ironed flat, I couldn't see the keratin bonds. But the next time I got a blowout, I got big, voluminous curls, which made the extensions less obvious.