I have never had a problem mixing up my fabrics. But then I grew up with true scrap quilts my great-grand and grand mothers made. You can start by deciding on a pattern and color(s). I like Depression block on Quilters Cache. If you are using say green, cut your squares and divide them into lights and darks then place in separate bags. Choose one from each bag and make 2 HSTs. When you have enough arrange them into the blocks and sew together. Once you are comfortable with a "controlled" scrappy quilt perhaps you can move on and use more and more colors. Nothing says a scrap quilt cannot be "controlled scrappy", your quilt, your rules.