Hi Holly; slipping the stitches knitwise when you're binding off for the buttonhole is just as you would do for an SSK decrease, and it makes for a neat, left-leaning bound off edge. If you slipped those sts purlwise, there would be some twist in the BO stitch when you pulled it over the next stitch, potentially making it look a bit wonky. In all the other places where you're slipping stitches, you do not want to change the orientation of the stitches but just move them from place to place, so just slip purlwise with leading leg in front.

herlittleway Did you ever get your question answered? I was wondering the same thing.

Try it by following the steps as depicted; it's very straightforward. The yarn position is exactly as described in the tutorial, and the CO is just a purlwise version of the knit-on cast on, as you might use to bridge a gap elsewhere. This cast on is used because it results in actual worked stitches (rather than just flimsy loops like you would get with a backwards loop CO) to close the buttonhole and make it stable.

It sounds like your pattern already has had you bind off 2 stitches on the previous row? So when you get to that place, you CO 2 replacement stitches and continue. OR you can try the method described in my video, which will give you a tidier overall result.