Toddler tank

Need help on decrease in neck
Pattern says: shape neck:RS facing, work across 18 st, turn leaving rem state on hold Purl back
Dec 1 st at neck edge eor 3 times

Do I do a k2 tog or ssk at end of row?

It’s really up to you. When you turn are you leaving sts for the center neck and the other shoulder on a holder? In that case you wouldn’t decrease on the purl row but at the end (the neck edge) of the next row. I use a k2tog here but you might prefer that ssk.
What pattern are you working?

Thank you so much for replying. I wasn’t sure how this worked since I’m not very good at computer skills.
I am knitting the Piñata Toddler Tank in a size 4. & I’m pleased with it so far. I’ve knit it in the round (bottom up) to the armholes & then put it on 2 needles for front & back. Now I have don on for sleeves but the decreases were getting to me? Had no idea which way to go so thanks a lot for your help.