There are a lot of good guides to designing sweaters out there. Books, blogs, websites, etc. Most of them say something similar: Knit a swatch, using gauge determine how many stitches to cast on. Design entire pattern out before knitting anything.
That's too much like work. I'm not saying that you shouldn't design a sweater that way; you probably should. It's just that I don't want to design a sweater that way. So without further ado, I give you
Sweater Design for the Lazy
1. Decide what weight yarn you want to use. I chose DK weight, Merino Style from Knitpicks.
3. Using the already-established patterns, get a rough idea of how many stitches to cast on.
4. Decide that knitting the sweater in pieces is unnecessary, and wouldn't it be cool if there was no finishing to be done afterwards? Ignore the voice in your head that says set-in sleeves can't really be done without seams.
5. Fiddle with the number of stitches to accommodate your pattern, shaping, etc. Make sure to always go up in the number of stitches, so you can be sure it fits.
6. Discover that the size 6 needles you had intended to knit the sweater on actually measure (according to your needle gauge) as a size seven. Decide that you'd rather knit it on size 5 anyway.
7. Despite going down a needle size, make no adjustment to the cast on number. Justify this with such statements as, "I want it to be snug anyway." or "I'm using more stitches than the original pattern, it'll fit."
8. Cast on your stitches. 182.
9. Knit for about twenty rows before trying the sweater on (as much as you can). Decide it doesn't really fit. Rip it all out.
10. Knitting it in pieces does kind of make sense. Add more stitches to your ultimate stitch count and start again, on the front.
11. Knit, past the bottom ribbing and begin the pattern.
12. Decide that the original pattern doesn't work. You like the idea, but it needs some tweaking.
13. Rip back down to the ribbing.
14. Spend a couple of hours on Paint, redesigning your pattern.
15. Start knitting again, this time really liking your design.
16. Ignore the little voice in your head that informs you that you still don't know how to do the sleeve shaping or the neckline.
When all sixteen steps are completed, you should have something that looks like this:
One sweater front, just about ready to start armseye.
Of course, most of the above mentioned steps could be skipped if you just knit a damn gauge swatch in the first place.