the cable twist is usually done in 1 row. you may work maybe 3 rows “normally”, then do the cable row. you will then work 3 rows normally. different patterns call for different frequencies of the cable row.
remember, if it says “cable 6”, you will put HALF that amount on the cable needle. so if the cable is 6 stitches wide, you will put 3 stitches on the cable needle.
to do the twist row using your example:
-purl 3 stitches to make the pre-cable background
-put the next 3 stitches (from the left needle) on the cable needle
-either hold the cable needle to the front of the work (for a left twisting cable) or to the back of the work (for a right twisting cable). your instructions will tell you which side to use. they will say something like “C6L” or “C6R”
-knit the next 3 stitches that are not on the cable needle
-knit the 3 stitches that are on the cable needle, using the cable needle as your left needle (in other words, you are knitting directly OFF the cable needle)
-purl 3 stitches for the post-cable background
that row will create the twist. if you work the next 3 rows “normally”, then do another cable row, you will see the cable emerge.