To knit a cable, you slip a designated number of stitches from the left needle onto a 3rd needle (a cable needle or DPN). You then hold the stitches on this needle as directed either in front or in back of your main knitting, while knitting more stitches normally from the left needle to the right. Finally, knit the stitches from the cable needle back onto the right needle.
Whether you hold the stitches on the cable needle in FRONT of or in BACK of your main work, will determine the slant of the cable. It is very important to pay attention to the pattern so that you are holding the stitches in the proper place at the proper time so that your cables turn out in the direction you intend!
A more detailed explaination of the cables in the Washington Square Scarf:
C8F: slip 4 stitches (1/2 the cable width) (as if to purl, so as not to twist the stitches) from the left needle onto a cable needle, and hold in FRONT of your main knitting. Knit the next 4 stitches (again, ½ the cable width) from the left needle, letting the cable needle rest in front of your knitting. Make sure your knit stitches (especially the first one) are snug. THEN: Knit the 4 stitches from your cable needle, using the cable needle as your left needle, while leaving the main left needle rest.
C8B: worked exactly the same, but holding the cable needle BEHIND your work.