Looking at a poncho in a store, I figured that it was just a big rectangle attached to a small rectangle. "Hey, I could make this at home," I thought. So here's some instructions about how to determine the size of those two rectangles. This isn't the only way to assemble a poncho, or even the best way, but it's probably one of the easiest. I'm not going to say anything about how you make those two rectangles; whether you knit them or crochet them or weave them or cut them out of fleece, that's up to you. But here's how you get them:
You only need to make two measurements:
1. The distance from your shoulder to your wrist (s-w on the diagram). If you don't want to make a full length poncho, make this the distance from your shoulder to mid-forearm. Or from your shoulder to your elbow.
2. The neckline depth (nl on the diagram). This determines the size of the neck opening. It should be big enough to get your head through, but not so big that the poncho will slip off your shoulders. To determine the smallest feasible nl, take the circumference of your head, then divide it by 4. Unless you're going for a really snug neck opening (say, to make a turtleneck), you'd probably want to add a couple of inches to that. 5 inches was what I used.
Here's a bad drawing of the poncho being worn, viewed from the front:
Rectangle A is really big. It drapes all the way around, hanging over the right shoulder (the wearer's left shoulder).
Rectangle B is much smaller, covering only the left shoulder (the wearer's right shoulder).
Here's the two rectangles laid out:
Rectangle A is the width of one shoulder-to-wrist length (s-w). It is the length of 2 s-w plus 2 nl.
Rectangle B is the width of 2 nl, and the length of s-w.
And then you sew the red seam to the red, and the blue seam to the blue.
Then you can add a finishing to the neck opening. Maybe a round of single crochet to just to tidy it up, or pick up the stitches with smaller needles and add some ribbing. Or knit in the round up 8 or 10 inches to make a turtleneck or cowl neck.
And that's it. Pretty simple eh?