I've just started on a true scrap quilt - I went through my "scraps" and cut the bigger stuff into pieces, down to 2.5" squares. Now I'm randomly sewing what was left together. I grab out a handful and start matching up edges and sewing them together. Once I get a bunch sewn, I'll press and trim so I have straight edge, and then start sewing the already sewing pieces together. Making some more square and some longer, I just keep that up until I've got big enough pieces I can cut 9" square out of.
I love making scrappy quilts, where i use an assortment of fabric. but the pieces are specific sizes. I try not to sew the same fabric together, but find that even if I am careful at the block level, once I sew all the blocks together I will end up with the same fabric touching somewhere (if I don't have sashing) so I don't worry about that.