Instead of using scrap yarn, a re-usable cast on rag comes in handy. Both scrap and a rag provide knitting to hang weights to make casting on easier. Especially useful when casting on a small number of stitches.
Creating a Cast on Rag

• Hang the strands of the rag on the needles in work
• Knit a row of ravel cord (manually or with the carriage)
• e-wrap cast on ~or~ continue knitting for open stitches
• Slip ravel cord out to remove the rag

Classroom:
Cast on Methods