Loud Sweater

Lou Sweater pattern

I am on the SIDES section now. So far, I only did k3, (k2,p2) till last 3 sts, k3. Then work the establish pattern till 6 cm after the CO 80.. Let me know if I did wrong.
Then, I really cannot understand the rest., and not sure how to make it work.

Looking for help/ advice. Thanks

SIDES
Using 3 mm needles, cast on 80 sts and work in k2,p2 rib for 6 cm, beg and end with k3.

Change to 3.5 mm needles and patt st (see STITCHES); evenly dec 10 sts in 1st row [70 sts].

On each edge of foll 6th rows, cast off 1 st 20 times, then on foll 4th rows, cast off 1 st 3 times.

STITCHES
K1,p1 rib - K2,p2 rib - St st
Patt st: 2 rows in rev st st, 2 rows in st st, repeat from * to *

After the decrease of 10sts across the first row, call the next row, row 2. Cast off one stitch at each edge on rows 7,13,19,25,31,37,43,49, etc to row 121. Then cast off one stitch at each edge on rows 125,129 and 133.

Does that mean I have to do the “stitches” pattern once I change to 3.5mm needles, and do the dec 10sts?
But I really don’t understand the stitches pattern listed., and how to do it?
STITCHES
K1,p1 rib - K2,p2 rib - St st
Patt st: 2 rows in rev st st, 2 rows in st st , repeat from * to *

Thanks

Work the decrease row in reverse stockinette stitch. The means start on a right side or outside of the knitting with a purl row. Then work a knit row. The purl bumps will show on the outside of the piece.
For your pattern stitch, the 2 rows in reverse stockinette are followed by 2 rows in stockinette. So 2 rows rev sts (purl one row, knit one row) followed by 2 rows st st (knit one row, purl one row).