The stitch pattern is on page 136 of Barbara Walker's A Second Treasury of Knitting Patterns.
Pick your yarn. I used Galway Highland Heather in a lovely grey. It's a worsted weight, 100 grams = 210 yards.
I cast on 36 stitches and swatched with #5 needles in moss stitch for 8 rows, then switched to #7 for the stitch pattern with 5 sts of moss stitch on each side of the swatch until the stitch pattern section was roughly square, then did 8 more rows of moss stitch with the smaller needles.
I blocked the swatch by soaking it, carefully pressing water out of it, then blocking it out with tig welding wires on a towel, safe from the teeth of my kitty cats (one of who has a personal vandetta against wool of all sorts).
After the swatch was dry, I unpinned and unwired it, gave it a good shake, then laid it out flat and measured the gauge.
My gauge was 5" for 26 stitches (or one pattern repeat) and 4" in 30 rows.
I then measured my back, from shoulder bone to shoulder bone, and saw that it is 15". That is three pattern repeats, luckily. This is a good-luck sweater, and you'll see more of why later.
So, I knew I would need 78 sts for the back of the neck. I wrote this down and launched into the small saddle-shoulders.
If you look closely at the pattern stitch, you will see that it is made up of two 13-stitch sections, facing each other as in a mirror. So, the very very narrowest piece of knitting you an make without completely rewriting the stitch pattern is 13 stitches across. I cast on 15 stitches, as I wanted two selvedge stitches I could slip at the beginning of each row to make a neat edge, and they won't make it very much wider at all.
I measured from where I wanted the neck of the sweater to begin on the side of my neck and the shoulder-bone. It is 5" or about 36 rows. So, I made two little strips of knitting, in pattern, 15 sts wide and 36 rows long. I cast them on temporarily and left the live stitches on stitch holders. You'll see why in a minute.
I then considered the front and back of the sweater. Coming straight down from the shoulder, I would need three complete repeats of the pattern, 78 stitches. I realized that making the pattern line up properly when the sweater was closed would mean splitting the pattern in half, visually, and being careful to work the right half on the right part of the front and the left...well, you know.
I would need 5 (moss stitch edge) + 13 (half a repeat) + 26 (a whole repeat) for the left front of the sweater. 36 of these I could pick up from the edge stitches of the saddle shoulder, right? Wrong. Because these stitches were slipped, the most I could manage was 20 (but don't ask me how, it should have been 18). So, I picked up 20 along these edges and then increased by six in the next row (that's after every fourth stitch, plus one at the beginning).
I wanted a square neck, so I picked up the 20 left-front stitches, increased to 26, then worked 12 rows in pattern. I did the same for the other side. Then I started the back by picking up 20, temporarily casting on 26 for the center back (those 5" again), and picking up 20. I increased those 20s to 26s and worked 12 rows. I made sure the next row was a wrong-side (purl only) row.
Next, I put it all together by temporarily casting on 18 sts, working the right-front stitches, picking up 8 stitches from the edge of the piece I had just made (the little ear near where the right shoulder-seam would normally be, working the saddle shoulder stitches from a holder, picking up again from the edge of the back, working the back stitches, picking up from the other side of the back, working the other saddle shoulder stitches, picking up from the next edge, working the live stitches from the left front, then casting on 18 more stitches. Whew. Now, the makings of a square neck, with a very rectangular top to the sweater. I placed markers where the sleeves were, and that included the picked-up stitches.
I increased once at each of the markers on pattern rows, leaving stitches, when there were less than 13, all alone in stockinette stitch. This is the same rate of increase you would use in a raglan sweater, if the body didn't have increases.
If you look closely, you'll see that I have the shaped areas of the sweater, like the tops of the sleeves and the waist, as stockinette. This did two things. 1) Kept me from going bonkers. 2) Gave me a little more leeway on my fit. Stockinette has a larger gauge than the stitch pattern, at least for me, so having 10 stitches in stockinette was larger than the same 10 stitches in pattern. This can be good for those of us who like to design skin-tight sweaters but don't like to wear them that way.
I worked 36 rows after this pick-up row, just increasing for the sleeves and leaving the body alone. Then, I began increasing the body stitches at the same rate, five times. Then I divided for the underarm, adding 16 stitches under the arms for ease. (Barbara, in her Knitting from the Top Down, said I could add up to 3" of stitches here. I took her at her word).
I arrived at this rate of increase by running a ruler down the length of the blue sweater I made last year, which is in a similar yarn, but is a set-in sleeve from the bottom, ala Elizabeth Zimmerman. I pretty much copied the rate of shaping.
My exact notes from this section of the sweater:
"work body straight until 6.5", begin increasing on RS rows until length is 7.5". Divide for underarm, casting on 3" of ease. Width here = 40". 40-6-30 = 4, need 2" in front to increase to an armhole - 10sts in about 8 rows, make it ten rows, increase by 2 every RS row".
Now you can see why I failed when I tried to interpret my notes on the fly in my podcast. Remember that you have to take into account half of the width of the saddle shoulder when figuring how long the sweater is at any given point. Also notice that I often say width when I should say circumference.
Anyway, what's next?
Divided at underarm, I took on waist shaping, which is that 4" below the underarm, the sweater should be 36" - in this case that means decreasing 10 sts in 30 rows. That's five sets of decreases about every 6th row - every third right-side row. It also happens to be every 1/2 of the pattern repeat, which happens to be very easy to spot in this sweater. So, no stitch markers for me, on the body anyway. See, lucky sweater.
Repeat, in reverse, to round out for hips.
Work 2 more inches straight, divide for "vents" on the side, work vents for 4", BO.
Sleeves - 14" to shape, 3" straight at end, top is about 14" around, bottom is 12", so about 2" or 10 sts in 105 rows. That's five groups of two decreases (again!) or one set of decreases every 20 rows. Then 20 rows straight, then 7 rounds moss stitch on #5 needles.
At the end, or halfway through, I picked up for the neck on #5s (remember all of those temp cast ons) and worked moss stitch, with decreases at the corners to make it lie flat for 6 rows, BO.
This is all for me and will vary with the knitter/yarn/size. Please comment if you have questions and I will do my best to help.