Beginners, FAQ & Quick questions

hello! I've started making a ridge scarf pattern which is made by front and back post double crochet stitches and so far I'm pretty happy with my work, however I've noticed that the edge of my work is wavy/ruffled looking at it from the side. I'm a bit lost because I've never seen this happen before, and the rest of my work lays pretty flat. It's not too intense but it's definitely visible. I've taken a picture of both sides:

I've used acrylic yarn for each of my projects but now I'm using mercerized yarn for the first time, with a 4mm crochet hook, recommended hook size is 2.5-3.5mm. The instructions also say, "work a double crochet into the top of the turning chain 3" and I was able to do that thanks to the tutorial but I wonder if I worked the end of the later rows into the right stitch.

I need help with figuring out what's most likely to be causing the ruffles. Should I change the hook size? Is it not the best yarn to use for this project? Can it be that I'm ending the row wrong? Is it something else?

/r/crochet Thread