How to knit this pattern?

I am knitting a Norah Gaughan pattern called "FLOW" and it is my first sweater.

I don't understand what I need to do with the following instructions:

Shape neck (rs): K 28, join another hank of yarn, and bind off center 16 sts, k to end

(How do I join the yarns?)

When you have knitted the first 28 stitches, get another ball (or hank of wool). Let the tail of the ball you have using up until now lie at the back of the work.

Using the new ball (and leaving a reasonably long tail, start knitting for the cast off. When you have cast of the 16 stitches, knit to the end of the row.

Before you start on the next row, turn the work and tie the two tails together so that your work doesn't unravel while you are knitting.

THEN:
Working both sides at once, bind off 3 sts at each neck edge once.

(What does it means both sides at once? How do you do this? After binding off the 3 sts, just knit to end?)

Working both sides at once means that for every row you do on the left side, you also do the same row on the right side.

For example with a right side row you would knit across the first side (the left side). Then you would pick up the yarn on the right hand side of the neck, cast off three stitches and knit to the end.

Assuming that you are doing garter stitch the next row(s) would be knit all stitches on the first piece. Pick up the yarn on the second side, cast off 3 stitches and knit to the end of the row.

THEN:

2 sts twice ;

(does this means that then after doing the first row with the 3 sts bind off, then I do another and I bind off 2 sts instead of 3 ?)

Without being able to see the pattern myself I am not 100% sure, but I think it does mean that you cast of 2 stitches instead of three.

THEN:

Dec 1 st at each neck edge every row, 4 times ( This means that I dec 1 stitch at the beginning of the row? and do this for 4 rows?)

The neck edge is at the end of the row on the first side and at the beginning of the row on the second side.

So you would knit to the last 2 stitches on the left hand side and then knit the 2 stitches together. On the second side you would knit 2 together and then knit to the end.

For the second row, you would knit to the last two stitches on the first side and then knit 2 together. On the second side, you knit 2 together and then knit to the end of the row.

Do these rows twice.

Then:

every RS row 5 times each side?

Does this means to do the following :
1st Row: Knit on WS,
2nd Row: K and dec 1 st on RS
3rd Row: Knit on WS,
4rd Row: K and dec 1 st on RS
5th Row: Knit on WS
6th Row: K and dec 1 st on RS
7th Row: Knit on WS
8th Row: K and dec 1 st on RS
9th Row: Knit on WS
10th Row: K and dec 1 st on RS

Yes, that is right. But remember to do the decreases at the appropriate edge.