Icelandic jumper

Hello, Im stuck at the shoulders /neck of the jumper Im knitting.
it says, K across remaining 11 stitches casting off at neck edge alt rows
3 stitches once and 1 stitch 6 times (=13 rows and 5 stitches to centre st of front) there are 12 stitches
at the front of the jumper which i`m knitting on circular needles. Anyone help please.

Welcome to KnittingHelp!
You’re knitting back and forth on the circular needles at this point?
You want to decrease at the beginning of the row on the neck edge (not the armhole edge). Cast off 3sts, work to the end of the row, turn and work the next row, turn and then cast off one stitch at the beginning of the row. Repeat the decrease of one stitch at the neck edge every other row 5 times more (6 times total). That’ll decrease your stitch count by 9sts and take 13rows to complete.
Can you give us the name of the pattern or a pattern link, please?

Thank you for answering my query. The pattern is Alafoss Lopi book 10 and
the pattern is Vor/Esja page 8.

maybe this pattern:

Hello, Yes thats the jumper, Im a bit stuck at the shoulders/neck.

What in particular about the neck shaping?

Hello, from the bit at the at the bottom of the page where it says "work
across 10 stitches leave remaining 11 stitches on holder " and then
"continue as set across remaining 10 stitches until centre stitch at back.
Leave on holder. Is it the 11 stitches on the holder I should be working
on for casting off.

Have you been working in the round up to this point? Or is this the back or front of the sweater knit in pieces?
How many sts to you have on the needle?
Sounds like there’s a center stitch marked at the back and a center stitch at the front. How many sts on each side, that is from the center back to the center front?
I don’t have this pattern so I’m trying to figure out where you are in the sweater.

Hello, Im working in the round. Im working on the right sleeve with
circular needles there are 21 stitches on the needle 20 stitches from the
sleeve and one stitch from the main needle for the shoulder decrease, there
is a marker on the centre stitch. I think the shoulder type is saddle
strap. Thank you for your help, Ill leave it for a couple of days and then come back to it, and maybe Ill see how to do it.