Pattern says, 'Cast off 5 sts, patt until there are 6 sts on right needle & turn, leaving rem sts on holder'.
Q) Which ones are the remaining sts?
A)CO 5, work 6. Everything left on your left needle are the remaining sts.
Q) When it says turn does that mean literally to swap the needles around?
A)Yes. You just did a rs row, now turn to do the ws row.
The next part says, 'Work each side of the neck separately. Work one row.
Q) Which stitches am I working in the one row?
A) The 6 on your right needle (which you have now turned so they are on your left needle waiting to be worked)
Next bit says- Cast off remaining stitches.
Q) Which ones are these?
A) The same 6. Turn work again so you are on the rs, and bind them off. Cut yarn, leaving a tail
With RS facing, rejoin yarn to remaining sts, cast off centre 13 sts, patt. to end. Complete to match first side, reversing shapings.
A) Put the "remaining sts" (those you didn't work which are currently on your holder) back onto the needle. Make sure they are on your left needle so you can knit across the rs.
Start knitting (in this case you BO 13) with the yarn you just cut. Work the remaining sts (Should be 11) so as to match the other side (knit 11, turn, BO 5, work 6, turn, BO 6, cut yarn).