A) Make sure you get the right dye for your fabric.
B) Make sure you get
enough dye for your fabric.
C) Make sure you rinse all the excess dye out.
D) If you can get it, use a dye fixative as well, or you might get bleed-off/rub-off onto other things. The fixative also improves how lightfast it'll be, as well as colourfastness. Once you've done that, fabric softeners can eliminate some of the post-dyeing stiffness.
Caveat: a true deep black is finicky, from what I can tell, with most readily-available dyes. I don't tend to use Rit or Dyelon or the like, so I don't know how theirs are (some bad experiences made me shy away from them), but most black dyes will have some cast to them. Just read through whatever you buy and they should clarify things. You may have to dye it several times to get it as true-black as you want.
Acrylic is a bitch to dye. I hope you're not working with that.
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