Casting off is the opposite of casting on: rather than putting stitches onto the needle, you're taking stitches off the needle, to finish your project. Casting off is simple:
1. knit your first stitch.
2. knit your second stitch.
3. using your left hand needle, grab your first stitch, and pull your first stitch over your second stitch, and off the right hand needle (slides 1 and 2).
4. repeat steps 2 and 3 until there is one stitch remaining on the right side needle.
5. cut your yarn end and pull it through this last stitch. Tighten it, and you're done.