Make sure the steek stitches (five of them, unless you tweaked the pattern I gave you), are bound off in traditional knit two, pass the first stitch over the second stitch. Then leave all the rest of the body stitches on the needle and secure the steek. I strongly suggest using either a crocheted steek or a machine-stitched steek. The other methods of securing steeks before cutting are a bit fragile and not suited to the handling this one will get while you finish knitting the neck. If you are working with anything other than wool, I suggest a machine-stitched steek.
Steeking is discussed in great and useful detail on Eunny Jang's blog, so click here for the directions for both methods and a couple others besides. If you want to skip straight to a certain technique, crocheted steeks are here, and machine-stitched are here. Actually, Eunny hand-stitches hers, but you can see straight off how to do the same thing with a machine; basically, set your machine to a very small, straight stitch, then sew along each side of the steek, and cut between.
Eunny covers the whole thing more beautifully than I ever could.
However, I've got a few thoughts and hints for cutting steeks.
First of all, pick out a single column of what I call 'cross-bars', the strands of yarn that stretch horizontally between two columns of stitches. The one slap in the center of the steek is, of course, the best choice. Then get a SMALL pair of scissors, and snip your way up the front of the sweater, one cross-bar at a time.
I prefer using a pair of very sharp, blunt-tipped kid's scissors. Nail scissors appear to be a favorite also. The only time I used a pair of sewing shears, I cut a huge hole in the chest of the sweater I was steeking. Something to keep in mind.
If, through some disastrous mind-meld with me and my nefarious influence, you manage to cut a float on the back of your jacket, just darn the ends in. (You can do that, once or twice. I do NOT suggest it as a regular way to create a knitted fabric.) Since the ends of the cut float will be short, do it like this:
Get a sharp needle and run it down through other floats and the backs of stitches, near where your cut float stops floating and becomes a knit stitch. Make sure that you put the needle THROUGH the yarns, so that the fibers and the twist of the yarn can hold the end you will be darning into it.
(Heeheehee, new camera with in-focus closeup, heeheehee.)
THEN, thread the needle with the short end of the float, and pull down through the yarns with the needle. Repeat for the other side of the float.
I suggest not cutting the steek until you're ready to do the shoulder straps and back of neck; steeks are unstable by their nature, and it's best to not leave them sitting around cut if you can help it. (Once the shoulder straps and back of neck are done, you can then immediately pick up the stitches all around the steeked edge and neck and knit the button band/collar, thereby stablizing the edge again.)
Next up, the shoulder straps and the back of the neck:
This is where you'll have to knit back and forth in two colors, but it doesn't last very long, it looks great when it's done, and you're invincible by now, anyway.