A cable instruction - CB6, or CB4 - has the total number of sts involved. You put half of them onto the cable needle and let then hang in the front of back while you work the other half of the sts. Just follow the instructions as written out - [COLOR=Black]slip next 3 sts onto cn and leave at back of work, k3, k3 from cn. Put 3 sts on the cable needle and hold in back, k the next 3 sts, then knit the 3 sts off the cable needle. That’s all.
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Like suzeeq says, you put 3 stitches on the cable needle [cn]. Then you move the needle + stitches behind your work. Knit the next 3 stitches from your regular needle, then knit the 3 stitches from the opposite side of your cn. [Here’s a hint that may help: use a spread-out bobby pin for your cable needle. Then when you slip the 3 stitches from your needle onto the cn, use the wavy-tipped end of the bobby pin to slide the stitches on. Then when you knit the stitches off of the cn/bobby pin, you knit them off from the smooth-side tip of the bobby pin.]
Cables can be confusing, but they’re tons of fun. Even your mistakes look impressive, often.
And once you get the hang of it, doing cables will quickly become second nature.