Casting on 24 stitches will make for a very skinny item. You're making a sort of basketweave type pattern, so you dont want to make it 12 garter stitches instead of 6...work what is written in multiples of 12...
So it would be k2, p2, k2, then 6 stitches in garter , then k2, p2, k2, six sitches in garter....and then do it again if you wanted 36 stitches and again if you wanted 48, and so on. Doubling in the middle of the pattern would make your "weave" patter very uneven.
The reason it doesn't say K8 instead of K2, 6 stitches in garter stitch is to separate the two sections of the pattern (I assume).
Hope that helps.
I'm Knittermom9 on Ravelry