Jenny, for my needlepoint cookies (and all my linework), I like to use a PME tip #1. I use a medium/stiff consistency, and I pipe away from me (bottom to top, not side to side) to get a straight line. The beads (dots) should be stiff enough to hold their shape without forming a peak. And I pipe the white dots first because it’s easier to correct any mistakes when the beads are the same color as the grid. There is a video on my IG page (@sweet_prodigy) that shows how I pipe lines (sorry, it’s not the best video). I hope this helps.
Sweet Prodigy
