Help for a beginner please

I don’t understand the following instruction for casting off - please can anyone explain for me? . . . .

‘Cast off.
PM at each side of the centre 21 sts for back neck’

A little more of that row will help, but it looks as though you mark off the center sts to cast off for the neck. Then you probably work the sts to the first marker, cast off with another end of yarn, and work to the end of the row.