I do mine similar to IrishRose.
--I make strata while the fabrics are WOF -- just sewing strips together and I don't usually care if I have the same width strips or not. If some strips are too short, I just add another strip to fill in the length (saw this on Simply Quilts -- don't remember who was on). Sometimes, if I feel doing extra work, I will stitch the strips together, but usually I just but the ends together.
--I usually make at least 2 or 3 different strata so I get a random assortment of strips. I cut the strips the desired length (though I add 1/2" extra not 1" extra). I skip over the area where the strips are joined when I make my cuts.
-- If the fabrics are non-directional, I'll turn the smaller strip sets upside down to make them more irregular.
-- I join the smaller strip sets together to get the length I want for the sides.
If my strips are different lenghts, I use corner stones instead of mitering the corners because my eyes like all the strips to match and irregular widths give me fits.