how do you purl into st below next st?
This isn’t something I’ve seen (not that I necessarily would…) is that exactly what the instructions say? If so, is it possible that in the book/pattern it describes the stitch somewhere? Often if a pattern calls for something a little unusual it will explain it somewhere before the pattern begins.
Or if it doesn’t give instructions, I’d put the tip of my right needle under the loop of the stitch below the first stitch on the left needle and purl into that.
Look at P-B on the abbreviations page.