I'm wondering if "D13" was actually "DI 3". It just makes more sense to me since it is supposed to be dec and inc 3 sts all at the same time. Anyway, it's fine as long as there is an explanation on how to to it.
Here is how to do "Slip 3 sts purlwise, slip those back to LHN; (k1, yo, k1) through the back of those 3 sts."
First, slip the next 3 sts to the right needle purlwise (insert needle to each st just as you're purling but don't purl, only pass them on to the rt. needle). Then slip those sts back to the Lt. needle. Knit 3 sts together through back loops, don't slip them off just yet, do YO once, and then knit those same sts again and then slip the 3 sts of the left needle. This is count as 1 DI3 or D13 whichever you want to call it.
For the next part, you can then do a set of (p1, D13) for 14 times. Then on the next row, p1, then do (k1,p3) 14 times, then k1.
I know it sounds complicated. If this is not helping you then let me know. I will make time to do a video for you.
Thanks for the question and have a happy new year!
The instructions are dec 1 stitch at neck edge...on alt rows AND AT THE SAME TIME cast off stitches at beg of 2nd row. I am confused between alternating