I need some help.
This is pattern:


This part i und.:
co 90 (100/110) sts using 3.5 mm dpns, place stitch
marker and join for knitting in the round
k 12 rnds (you can try and use circular needles after a
few rounds, it’s much faster, use magic loop method
if you got a long circular needle)

This part i dont und.:

p 1 rnd, k 13 rnds
fold at purl round (with knit side facing and purl
side inside the double blending), pick up first st
from cast on edge and k together with live st, repeat
this for making a double edging of hat
knit 1 rnd, then switch to 4 mm needles
inc rnd: *k10, m1 *repeat

Begin main pattern i und too.
So its just that one part.

This video may help:

At the 3 minute mark, she demonstrates the hem. The video includes a picot hem but your pattern has a purl row as the turning row. Then follow you directions to knit 13 rows and then see the video for joining the cast on edge to the live sts.

What that m1 mean?

inc rnd: *k10, m1 *repeat

Most patterns will explain the abbreviations. See if yours does.
M1 means make 1 and is an increase. There is a video for it in the free videos at fhe top of the page.