I'm not a braider, but I do have my hair in cornrows, minus the extra hair. Here is what I find helpful that may work for you. If you are going to use synthetic hair, then I would probably rinse the hair to get the "chemical" that they use to treat the hair out first. Let it dry for a day, then proceed to cornrow my hair. However, I would use a nice moisturizer on my hair first (Hawaiian Silky 14 in1 - thanks Tiffers!!!), so that my hair is not dry and will not break during the cornrowing process. The is a website, and excuse my ignorance for not knowing the exact web address, that states how to rinse the synthetic hair before using it on your own.
perhaps one of the ladies can share which website. i know this website uses the crown and glory method.
http://public.fotki.com/Arco-Jamaa/notsonatural/ try this link she put in some lovely canerows with extensions and shows you the process step by step. I copied the style with good results pic is in my avatar.