I learned to sew before I learned to knit. At first I tried handsewing with my mom and grandmother. When I was nine, my mom decided I was ready to give the sewing machine a go. Brilliantly, she handed me over to our teenage babysitter Robin C. who lived across the street from us. Both Robin and her sister Ellen sewed beautifully and although my mother was a wonderful seamstress herself, she knew it would be much easier if someone else, not her, taught me to sew. And besides, we Nicholas Girls idolized those Cashen Girls. My first project was an a-line mini-floral print dress with a lilac faux tab front and a zipper in the back. I wore that dress in my 4th grade picture and mostly likely until I couldn't squeeze into it anymore.
So what, you ask, does this have anything to do with steeks? If you haven't guessed by now, I went on to have a long sewing career making most of my clothes through my teen years and beyond. When I eventually learned to knit proficiently, I was in college majoring in Textiles and Clothing. Knitting was something I picked up for fun. I could do it on the train or the bus going back and forth to college.
Eventually my interest in knitting began to take over my sewing hobby. I started spinning wool. And then I had all this yarn which I couldn't find a pattern for. A professor of mine wisely recommended some books including Elizabeth Zimmerman's Knitting Without Tears. I started designing my own patterns for my own handspun yarns following the sage advice of the wonderful EZ. I noticed the word steek but I was into "knit in the round" sweaters. A year or so later, I picked up a book called Knitting in the Nordic Tradition and saw the steek technique again. I decided to give it a go and haven't turned back since. I knit most of my sweaters in the round and cut them and sew the sleeves in. I'm not afraid and you should not be either.
When I was working on Kristin Knits and I got to the section on sweaters, I thought long and hard about "knitting in the round" vs. "knitting back and forth." I have written patterns both ways. When I worked for TYC, I always wrote my patterns for back and forth knitting. We didn't feel our shopowner customers were interested in "steeking." We were afraid we would turn them off to certain knit in the round and steeked patterns and then sit with a warehouse full of them. We realized the LYS-owners didn't have a lot of extra time in their day and wouldn't want to be bothered with helping their customers learn about and make and finish steeks.
But my book was my own thing. I could do whatever I wanted, within reason. I asked a lot of knitting friends what they thought. I asked some shopowners. I was torn. In the end, with the decision making help of my wonderful editor Gwen, I decided to present two of the five sweaters included in the book to be worked in the round with steeks. The photo shown here is a steeked sweater from Kristin Knits. (The lovely alternative photo was not used in the book and it is by Kevin Kennefick.) This sweater has steeks at both the neckline and the armholes.

I've taught a lot of knitters how to sew and cut a steek. I think the first thing to remember is that you must have no fear. If you are very timid, practice on an old swatch following the directions below. I promise you it isn't hard - it is mostly your fear of the unknown that is holding you back!
Enough of the chit-chat. Let's move on to how you do it.
First of all, what is a steek? Briefly, a steek is an extra set of stitches knit into a garment which will become a seam later on. A steek makes it possible to knit sweaters in the round (or anything for that matter) that later needs to be seamed or have a zipper or edging added to it. I knit my steeks in alternate colors (knit 1 dark, knit 1 light) so that the yarns are caught into the fabric and that it will be sturdy. On the next round, I swap the colors and a little check fabric will develop. (I also knit steeks on solid color sweaters so that I am always working on the right side but I'm not going to talk about that here.)
The biggest advantage of including a steek in your knitting is that you will always be knitting in the round. If you have dyslexic tendencies like I do, it is much easier to always see the right side of the knitting and follow a chart always from right to left. I can't be bothered with working colorwork back and forth and I even knit most of my swatches in the round on double pointed needles.
Here is a steek at the armhole of the sweater. Note that the steek begins after the fabric has been knit in a regular pattern until the armhole needs to start.

Here is a steek done at a neckline. You will notice a little pouch created by the bound-off stitches at the base of the neck. The gentle side neck shaping occurs at either side of the steek.

I use my trusty old Bernina sewing machine to stitch my steeks before cutting them. In these photos, I have used white thread. Set the machine to a straight stitch and sew between the stitches on the outside of the steek stitches. I call this "stitching in the ditch" and the machine stitching will actually disappear. Do this on either side of the steek - there will be 1 row of straight sewing machine stitches on either side of the steek.

Next, set your machine to a medium zig-zag stitch. My steek has 6 stitches in it. I am going to sew down the center of the two centermost stitches in the steek, making 2 rows of zig-zag. Locate stitch number 3 and sew a row of machine stitches over the the knit stitches. Next, locate stitch number 4 and sew a row of machine stitches over the knit stitches. The photo below shows the second row of zig-zag being stitched.

It is important to keep the knit fabric flat and neat while stitching. Do not pull on it as you sew or it will distort and ripple. Make sure you block and steam your knitting before doing the sewing machine work.
Now, take a deep breath and using a sharp pair of scissors, cut through the center of the two zig-zag rows of stitching. Be careful not to snip the sewing machine stitches (although nothing much will probably happen if you do). Be careful when you come to the end of the armhole steek so you don't inadvertedly clip the main part of your sweater. The following photo shows an armhole steek being cut.

That's all there is to it. Give it a try and see what you think. I have never had a steek fall apart and almost every sweater I have made has used steeks.
This series of photos was taken as a guide for the illustrations I did on Cutting a Steek in my new book Kristin Knits. I give full instruction in that book but as someone said, a picture is worth a thousand words.
If all else fails and you are still too timid to try, I suggest printing out this blog post and taking it to a local tailor or alterations expert along with your almost finished sweater. They will be able to do this for you quickly and easily and then you can get on with sewing the sleeves in and attaching the neckband and cardigan band.