I use it the way the very original instructions said to, although I've added a shampoo after it and a moisturizing rinse-out conditioner.
The original directions used to be:
DIRECTIONS: Shampoo with the NEXXUS shampoo for your hair type. Apply an ample amount of EMERGENCÉE to completely cover the hair. Comb through. Cover hair with plastic cap. Leave in for 10 minutes with or without heat. Rinse with lukewarm water. Towel dry. Massage a small amount of NEXXUS HUMECTRESS ULTIMATE MOISTURIZING CONDITIONER into the hair to maintain moisture balance. Proceed with styling. EMERGENCÉE is an excellent pre-treatment for delicate hair prior to all chemical services.
There was a discussion on changing instructions on the bottle, hence my adding a shampoo just in case. Here's that discussion:
http://www.longhaircareforum.com/showthread.php?t=39123