Selecting my fabrics for this technique is pretty easy and a lot of fun because any bloopers blend in so well in the later stages. I generally start with a range of lights, mediums, and darks in the color or colors I have in mind for my finished piece. It is easiest to assemble the collage if the pieces are cut into smallish rectangles, although with a little experience you can use any shapes you want, as raw edges ARE allowed. Above, I assembled my cut fabrics on the crinoline to which I will attach them. (Crinoline was discussed in a past post this month - nice firm backing and easy to stitch through by hand later in the process.)
Next step - making all those different colors and patterns look unified.