Help me with my sweater please

I had to rip out quite a bit because something went way wrong. Instructions say:
Once I get to 19 inches on the front of the sweater…
p37, bind off 16, p 37 stitches.
Decrease row - knit to within 3 sts. of bound of sts.
k2tog, knit 1.
JOIN a new ball of yarn to sts on left needle (HOW DO I DO THIS EXACTLY??)
(you will be working both shoulders at the same time) knit 1, ssk, knit to end of needle.
Next row - p each shoulder with its own strand of yar.
Work these last two rows 5 or more times until there are 31 sts. left on each shoulder.

So I tried all that and I’m not sure I joined the new ball of yarn on correctly - I just winged it.
Then I did as it says… work last two rows… so first row would be purl then the next row would be knit 1, ssk, knit, next would be purl and so on.
Well, I ended up with only 16 stitches on one side and 24 on the other. What on earth did I do wrong???
I ripped it all out and started over so now I’m back to the beginning of joining the new ball of yarn
Can anyone help me???

To begin with you should have about 90sts on the needle. P 37, BO 16, p37. Then decrease a stitch on each side of the neck edge (the bound off sts) on the knit row. To join new yarn, just start knitting with it. You dec each knit row after that 10 more times so you should have a dec of 12 sts for both sides, 6 sts for each shoulder, leaving 31 on each. Maybe you decreased on the purl rows too? Or you didn’t start with the right number of sts.

To join the new ball of yarn after you have bound off the neck opening stitches just start knitting with the new ball of yarn, leaving a nice tail. You don’t do anything special. It may seem loose at first but you can correct any looseness when you work in the ends. Make sure right after you do the bind off row that both sides end up with the same number of stitches. If they don’t redo it until they do.

The two rows that you will be repeating are the decrease row and the purl back row.

Decrease row - knit to within 3 sts. of bound of sts.
k2tog, knit 1. Then leave that side and go to the other side of the neck opening and knit 1, ssk, knit to end of needle.

You are decreasing one stitch each decrease row on each side of the neck. K2 tog decreases one and SSK decreases one. So the sides should stay even. You should count the stitches on each side after each decrease row if you are having trouble and make sure they are the same after each decrease row.

Next row - p each shoulder with its own strand of yarn. So that row you just purl each side back with the yarn on that side of the sweater.

I sometimes find it harder to work both sides at the same time (it’s not harder, but it confuses me sometimes, others find it the best way) and choose to just do one side at a time where I can really pay attention to that side. But still you need to make sure they are going to turn out even, and the same length.

You evidently decreased more on one side than the other. Be sure to only decrease one stitch on each side each decrease row—the knit row, and don’t decrease any on the purl back rows. Keep careful count and pay close attention. Also check the videos if you have any question about how to do the SSK, make sure you are doing both your decreases the right way so that they each decrease one stitch and only one each time.

Stop decreasing when you have 31 stitches left on each of the shoulders, 16 and 24 are each too few stitches for what it said.

ok I think I got all that. In my instructions when they talk about the SSK, it says slip one as if to knit and slip the next as if to purl then it goes on.
Every SSK video I’ve watched shows the two slips as knit slips. does that make sense?
Which one do I go by?? Or does it matter?

I slip both stitches as if to knit. When you slip a stitch purlwise in a decrease, it twists it, even if it’s under the other stitch. Some people like to do it that way as it tightens up the stitches, but if you don’t stretch out the first st too much when you slip or pass it over, you don’t need to tighten them.

I like to slip the second stitch as a purl.