Devide neck for jumper?

41sts on row

Rs k15 turn and leave rem sts on holder
Work east side separately
Dec 1 st at beg of next row and at edge on 3 foll rows, then on 1 foll alt rows 10 sts

Work 3 row end with rs facing

Am I meant to turn and put the 15 on a holder?

No, you are supposed to knit 15 and keep them on the needle, and then put the remaining 26 stitches on a holder, then turn your work and continue knitting on those 15 stitches that are still on your needle. After you finish that section, your pattern will probably tell you to start working on the stitches that are on the holder to do the other side of the neckline.

Hope this helps!