You can thread a piece of yarn with a tapestry needle through the stitches for the holder.
For the crotch, it looks like the four stitches on each dpn are the ones that get woven together.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry