The rattail I purchased came on a card so it has folds in it. How do I straighten it?

KVW

You might try hanging it out w/ weights...?  Either make a loop or put a clamp at the top end, hang it over a door jamb or door hinge, & then attach another clamp or hemostat w/ weights at the bottom.  Let rest.  I sometimes buy stacks of quarter-sized magnets & attach them as weights since they stick together so easily.

sher

 You can gently wet rattail and the kinks will come out of it (by gently, I mean just dip it in water - not scrub it or wring it out).  It can also be steamed, but wetting it is faster. 

Note: the real rattail cord is a rayon/cotton blend.  The other type of satin cord is nylon.  It can be a tad bit more stubborn for getting kinks out - you may need to soak it longer.  

And if anyone else ever needs the info, rubber cord can be dropped into boiling water to get kinks out.