To straighten frizzy hair with a flat iron, first wash your hair and towel blot it dry. Then, part your hair down the middle. Massage a product like Chi Silk Infusion through your hair by first applying a little bit to your palm and then working it into your hair. Use a flat rectangular brush and blow dry your hair. Separate your hair into sections. Use a flat iron and a comb to straighten your partitions of hair. Tilt your hair straightener at an angle when straightening the ends of your hair to make it extra straight. Use a good shampoo and invest in satin pillowcases to retain moisture in your hair. Finish your hair with an effective hair lotion to boost your hair's shine.