Casting on mid-row

Hi this is my first question, so hope I am following the correct format.I am knitting a simple baby sweater…one piece…My problem is that I need to make the neck hole. The first part is easy…knit x stitches, CAST OFF x, then knit x to end of row. TURN and knit x, then CAST ON x and then knit x to end of row…thereby creating a neck slit…
I can’t figure out how to cast on mid row…it’s a mess…can anyone help? many thanks…

I’d use a cable or knit cast on.

Backward loop/simple cast on would be OK if you were only casting a very few.

I vote for cable cast on; it’s much sturdier than backward loop, and looks a little better than knit.