Smooth and glossy are probably what you're going for when you straighten your hair—not frayed, dry strands with a bunch of flyaways. When your hair absorbs moisture from the air, the cuticles lift and makes hair look frizzy. Knowing how to treat hair with the right products after straightening it is the key avoiding frizz.
Spray a detangling product into your hands. Rub your hands together and run your fingers over the frizzy parts of your hair. The detangler smooths down the hair cuticle and eliminates the frizz.
You can also spray a shine or straightening serum on a flat paddle brush and run the brush over frizzy hairs to smooth them down.
Otherwise, avoid combing or brushing your dry hair after straightening it—this frays the hair cuticle and may trigger frizz. Finger comb your hair instead.