My Classic Elite Waterspun poncho is done!  Here it is mid-block:

I am really loving the improvised blocking wires I got to finish my Forest Path stole.  I threaded them through the I-cord edges, two per side, then spread the poncho out on some towels to dry.  The edges are straight, the corners are perfect, and dreaded curl-up has been eliminated.  And I didn't even need pins! 

And here's a shot of it on the happy recipient:

For those of you who wrote to ask for the pattern, here it is.

1. Make swatches until you get a fabric that you like.
2. Figure out your gauge.
3. Measure how big a neckline you need to go over your head.
4. Multiply that measurement by your gauge, and round it up to the next multiple of four.
5. Cast on that number of stitches, placing 4 markers evenly spaced.
6. Knit 6 rounds.
7. On the 7th round, K1, make one, knit to one stitch before the next marker, make one, K1.  Repeat this three times.
8. On the 8th round - knit
9. Repeat steps 7 and 8 until your poncho is as long/wide as you like it.
10. Bind off, or work one or more rounds of attached I-cord to finish.
11. If the neck is too wide, pick up purl bump stitches at the base of the rolled collar.  You should have the same number as you cast on.  Put 4 markers in your work evenly spaced.  Then work one round starting with k2 tog, (k1, p1) until you have two stitches left before the next marker, ssk.  Repeat between the other markers.  Then work a K1, P1 round continuig ribbing as established.  Continue this way, alternating decrease rounds and plain ribbed rounds for about 6-8 rows.  Bind off VERY LOOSELY, making sure you can still get your head through the hole.

As to what yarns are suitable, what number to cast on given a particular gauge, what size needles to use, how much yarn you'll need - this is all up to you.  Experiment!  Here are some thoughts to keep in mind:

What yarn is suitable?  Look at it.  Will it feel good when worn?  Is it hand-wash only, otherwise hard to clean, or a light color?  If so - are you prepared to care for it when it gets dirty?  Is it loosely or tightly spun?  Loosely spun yarns are more prone to pilling, catching and looking "used."  On the other hand, they're often softer with a more luxurious drape and sheen.  Is it heavy?  A poncho is a big thing - larger than a lap blanket.  Lift about 10 skeins worth.  Can you envision yourself dragging around that much weight (or more)?  Will the yarn stretch under its own weight when used in a large quantity? Cottons are heavy yard for yard and are infamous for this.  Wool is less weighty per yard or meter.  Wool/acrylic blends are lighter still.  Is the color/texture not only attractive on its own, but will it look good on you?  A puffy or furry yarn will add bulk and increase the size of your sillouhette.  A giant-gauge or shiny yarn will make a garment look larger than it really is.  Some colors and textures look fantastic as accents, but applied over an entire garment may not be as appealing on every wearer.  Decide what's important to you and choose accordingly.  Remember, you can always buy one skein to try out before you commit for the whole project.  If that yarn doesn't work out for a poncho, one skein might make a nifty hat or scarf.

What needle size to use? When swatching with a new yarn, start with the needle size recommended by the yarn maker.  Do up a good size swatch in your chosen texture stitch or colorwork design.  Do you like the feel and drape?  If so, measure your gauge - you're good to go.  Swatch feel too stiff and tight?  Try again on a size larger needle.  Swatch too drapey and holey?  Go down a needle size and try again.  You'll know when you've hit the best combo.  If you're combining several yarns of different weights or textures, be sure to swatch them as you will use them, using the stitch and needles you intend for the final project, even if that means making a VERY large swatch with multiple stripes.  Once you do get the look/feel you like, make a note of your needle size and FINISH  YOUR SWATCH.  You'll need it to do both gauge measurements and yarn consumption estimates.  (I'm not good at keeping paper notes, so I make knots in my dangling tail end to help me remember what needle size I used to make my swatch.  For example, four knots = US #4 needles.)

How many to cast on?  Simple math.  If your gauge is 4.5 stitches per inch, and you've decided that a 24 inch neckline is big enough, you start with 4.5 x 24 = 108 stitches.  That lucks out because 108 is a multiple of 4, and you don't need to round up.

How much yarn will you need? You can figure out roughly how much yarn your gauge square took for that number of square inches or centimeters.  Draw out a diagram of your project (in this case - a big square), and estimate how big you want the thing to be when it's done.  Figure out its total area and divide that area by the area of your swatch.  Got a 6-inch square swatch?  Want to make a peice that's 4 feet on a side?  4 feet x 4 feet = 48 inches x 48 inches = 2304 square inches.  6 inches x 6 inches = 36 square inches.  2304/36 = 64.  It will take you about 64 times as much yarn to knit your 4-foot square piece than it took to knit your six inch square.  Ravel back your swatch and measure, or weigh it to determine the amout of yarn you used.  Now do the math.

Shortcut:  If you like a slimmer poncho than this super-easy square one, there's a poncho pattern generation utility available elsewhere on-line.