Help with shaping a shoulder!

This is the first time I have knitted a vest - all was going well until this! Any help would be much appreciated!

Shape shoulder: Cast off 7 sts beg next row.

Purl 1 row. Cast off rem 7 sts. (do I cut the yarn after the final cast-off??)

With RS facing, slip next 25 sts onto a st holder.

Join yarn to rem sts (stitches from the stitch holder?? or the other shoulder piece??) and knit to end of row. (was the yarn cut?? or do I keep the yarn intact and go around stitches on the stitch holder??)

Dec 1 st at neck edge on next 3 rows. 13 sts. (this I actually understand!)

Knit 1 row even.

Keep the 25 sts on the holder, they’ll be the neck stitches. You can cut the yarn after you’ve bound off all the sts on the one side. Then take the yarn end and work the stitches on the other shoulder.