Help with pattern please.

I have just finished the body of a sweater Heilo-Falk 7902. I am suppose to work a new pattern that consists of 18 stitches beginning at side marker working up to and including center front stitch. Then working mirror image up to next side marker, stop and begin again as shown on graph at the same time cast off 23 center stitches of the front for neck opening. Work back and forth rounds begin at neck opening. I am really confused

It seems like you’re at the split for the neck so it makes sense that you might start working back and forth here (in the absence of a steek). Looking at the photo however, I don’t see the cast off of 23 sts.
Can you quote the instructions exactly for the round with the cast off? Don’t give us the whole pattern as that would cause copyright problems.

Now begin pattern 2 (it consists of 18 stitches), begin at side marker as shown on graph work up and including center front stitch than work mirror image up to next side marker. Stop and begin again as shown on graph and at the same time work front neck opening. Front neck opening cast off 23 center stitches for front neck opening. Work back and forth rounds begin at neck opening

Work the pattern across the front then it seems that you begin the graph again across the back. You could mark the center 23 sts at this point. When you come around to the marked center sts, bind them off and continue around to the front gap. From now on you work back and forth.
I’m not sure if this helps with the confusion. Come back if it’s still not clear. Take it step by step and see how it goes.

Okay so I knitted around and casted off the 23 stitches on the center front and ended back on the side. Luckily the first round was just one color. I am taking it that now I cut my yarn and start at the beginning of the cast off stitches but can’t figure out where I am starting pattern 2. Do I count back from were I started on the side? I really appreciate your time and help.

The pattern is indicating that you should work back and forth from this point on. Don’t cut the yarn. Turn as you would at the end of a row and purl in pattern across the wrong side row. You may need a new end of the contrasting yarn but the main color is in place.
It’s a complicated colorwork pattern but it’s going to be beautiful.

Think I got it thanks will have to end yarn when hit neck opening.

I think what is confusing me is it says rounds begin at neck opening

It should say rows since this is worked flat (back and forth) on either side of the front opening.

Stuck again on sleeve. Incease 1 stitch at the beginning and end of every third and fourth round alternately always leaving two stitches between the increase stitches

You’re going to increase on rounds 3,7,10,14,17,21,24,28 etc until you reach the given number of sts.