How to pick up stitches along side edge

I am knitting a blanket using pattern from Plymouth Yarn

Woolcotte Baby Blanket #F646

Loosely cast on 84 sts.
Section 1: Work in garter stitch for 4.5” ending with a WS row. (first row is RS).
***Leave sts on the needle.
Section 2 & 4: At beginning of next row cast on 16 sts.
Begin the RS row as follows: Knit 15 of the new sts, k2tog (one of the new sts and the first of the original sts). Turn, knit back.
Row 3: Knit 15, k2tog, turn.
Row 4: Work back (16 sts).
Repeat rows 3 & 4 until all of the original 84 sts are used up, ending with row 3.
Last row 4: Bind off 16 sts, Cut yarn.
Section 3 and 5: With RS facing reattach yarn to first row of 2nd section worked. Pick up 84 sts (one st for each ridge) along side edge of section 2 & 4. Beginning with a WS row, work in garter st on these 84 sts for 4.5”, ending with a WS row.
Repeat from *** once more. There are now 5 sections worked. Bind off remaining 84 sts- LOOSELY. Weave in all ends.

I am stuck on Section 3 - How do I reattach yarn ?what do they mean to "pick up 84 sts ?hhow do I do that?

Any help is appreciated .

Thanks


This is a video of how to pick up and knit.