Is This doable
 

Am I missing a step. I have quite a collection of mostly fat quarter (black & white). I'm thinking about using them as a backing. I will cut them into 18 inch squares, then nine patches and so on till it's big enough to make a backing. Is it that simple.

Sounds like a good plan!

Sounds doable to me. You could even cut them 18 x 21 or 22 (depending on FQ size) and use pretty much all of the fabric.

I've done that with smaller squares, 10" or so, but also made rectangles out of FQs, 1/2 yd pieces. I usually trim them all the same size and stagger them like brickwork. Sometimes I do the whole back that way. Other times I add a pieced section of brickwork between yardage.

I've been making double sided quilts for at least 20 years, maybe longer. Go for it, you'll love it.

black and white always makes for a stunning quilt. Anxious to see when finished.

Sounds nice. Go for it!

Problem I have had with pieced backings is getting it lined up straight. I use the Sharon Schamber method of basting. Maybe I will have to try spray basting.

a great idea, and I would use the whole fat quarter,as quiltsRfun suggested!

It is doable but I usually look at what is going to be the best fit for the back of the quilt. Those 4" your are trimming off might just help you extra yardage you need for a backing. My next quilt might just be this quilt that needs black & white prints called Diamond Heads by Cozy Quilt shop:
http://www.ebay.com/itm/172010101311...%3AMEBIDX%3AIT