If you have good hair, aka real remy from a reputable vendor, then you always wash the hair before your install. If you have regular hair from the beauty supply store then you do NOT wash before your install. I normally wash the hair in the sink. Fill the sink with water, add some shampoo and wash like you are washing a shirt - lol. Rinse and condition for few minutes, comb through (you should be able to comb through it easily at this point - if it tangles then you have gotten a bad batch and it will tangle once installed as well), rinse conditioner out, and let it hang dry. HTH
Disclaimer: The good hair comment is by no means a judgment on anyone's real hair - lol. Just differentiating between remy and beauty supply store hair.