Help me understanding this instruction

Need help with this instruction:K21 STs, slip center 2 STs to a stitch holder for a front neck, join a second ball of yarn and work to end of row -21 each side. ( THIS I UNDERSTOOD). working both sides at same time with separate balls of yarn shape armholes same as for back.
AT THE SAME TIME decrease 1st each Neck edge every other row 1 times , then every 4th row 5 times -9 stitches each side.

My question is after dropping two stitches on neck holder in that row . What should I do next n row and after that? I can’t understand decrease 1st neck edge instruction

Welcome!
Put the two sts on a holder, join a second ball of yarn and work to the end of the row. Now you have the two shoulders on the needles and a ball of yarn connected to each.
Turn and work across one shoulder to the gap where the two sts were, drop the working yarn, pick up the first strand and work the first shoulder.
From this point on you want to continue the armhole shaping and when you get to the neck edge (or maybe three sts before the edge), k2tog, knit the last st, drop the yarn, pick up the second strand, k1, ssk, and finish out the row. Keep doing the armhole shaping and at the same time, dec at each neck edge every fourth row for the given number of times (5x).