I’ve not blocked anything before since I’ve knit mostly soakers and longies. But now I’m working on the Branching Out scarf from Knitty with a 55% cotton/45% acrylic blend and I’ll need to block it when it’s finished. I know straight acrylic does not block well, but is there enough cotton in this yarn to make blocking worthwhile? How do I do it?
Thanks for any help!! 
I want to preface my reply with a note that I am a new knitter, although every second I am not at work , I am knitting!
I just blocked some sleeves (I knitted them first and wanted to see the post-blocking result) that are 100% acrylic (it is the yarn the pattern called for, ultra soft, and I am still leary of swadling myself in wool that I am still warming up to against my baby soft skin).
First, I wetted a towel and layed it on the ironing board. Then, I layed out the sleeves and pinned them into shape/size. Next, I layed another damp towel over the whole mess and left it over night. The next morning, I had PERFECT sleeves–flat as a pancake.
Maybe my technique will work for you! 
From reading in here for a few months I’ve learned that you can’t really block acrylic or cotton. Well, you CAN, but they won’t hold their shape so it’s basically a waste of time. Most acrylics can be machine washed and dried so that is the best thing to do I’m told.
It’s a lacy pattern, blocking is a good idea even though it’s cotton/acrylic. Here is a good article:
http://knitty.com/ISSUEwinter02/FEATdiyknitter.html
Good luck!
Lana told me for the gown…
take the steam iron, and hold it thisclose to the garment in question, and steam it…BE VERY CAREFUL to not let the iron touch th garment, as it will melt for sure…
(I’ll let HER tell you how she knows that…lol…)
The gown’s wrinkles alllll went away!!!