Help! Crocheting a neckline with decreases.

I’m currently crocheting a cardigan.
The pattern for the back says:
Chain 48/50/54/58 ch, then continue working in the basic pattern. At a total height of 11/13/15/17 cm, mark the edge stitches for the armholes. At a total height of 21/24/27/30 cm, for the shoulders, decrease on both sides in every row: 1x6 sts and 1x7 sts/1x6 sts and 1x7 sts/2x7 sts/1x7 and 1x8 sts. At the same time, for the neckline, leave the middle 22/24/26/28 sts unworked. Finish the second side of the neckline as a mirror image.

I chained 54 ch, chose a total height of 15 cm for the armhole, then continued crocheting until I reached a total height of 27 cm. By the way, the pattern is one row of double crochet, one row of single crochet.

And now I’m completely stuck… How is the decrease supposed to work?? I’ve never been so stumped before.

Who can help me? 

Hi Tanja
I’ll try to explain it to you.
At a height of 27 cm, you have a total of 54 stitches.
Now, on the right side of the work, crochet 14 stitches, then turn and crochet only 7 stitches. Now work a right side row with the last 7 stitches. This completes the right half of the piece.
The middle 26 stitches of the piece are simply left unworked—that’s the neckline. Now you need to crochet 14 stitches on the left side of your piece. Turn, crochet 7 slip stitches, then 7 stitches, and the row is finished. Turn again and crochet 7 more stitches. Then this side is finished too.

I hope I’ve explained it in a way that makes sense.
Here’s a little diagram to go with it.

Thank you so much for the explanation. I think I understand the principle now.
This is what mine looks like in the photo. Is it because the rows alternate between double crochet and single crochet?