The one I am wearing is done with size 13 needles, the one to the left is on 11's, and the blue one is on 15's. this is all up to you, as it just makes the poncho a thicker weight with the smaller needles. I used Homespun yarn because I love its softness, but any bulky weight will do, as I used no guage for this.

Sorry, not many pictures.
to begin, cast on 44 stitches for a neckline that is close to the neck; I cast on 56 for mine, as I don't like anything too tight around my neck. Join in the round, careful not to twist the yarn, place marker for beginning of round. I used two different colors for this, and I'll tell you about it later on.
Cast on 44-56 (must be divisible by four!)

Rounds 1-4, K2, P2

Round 5, knit, placing markers evey quarter (Cast on 44: K11, place marker, K11, place marker, K11, place marker. Casting on 56, I placed a merker after every 14th stitch)

Round 6, K1 make 1, knit to the stitch BEFORE the next marker,K1 make 1; slip marker, repeat ending with a K1 make 1 on last stitch. You are adding eight stitches on these rounds, one before and one after each of the four markers.

Round 7, K all around

Round 8, K all around

Repeat the last three rounds until the poncho is as long as you want it, changing to longer circular needles when needed. Mine was 26 inches from the bottom of the K2 P2 section to the bottom of the poncho. Then bind off, weave in ends and you are done!

This one is for my three year old granddaughter. I used size fifteen needles, and cast on 44 stitches, ending at 17" total from cast on edge to end. This fits over my head and is not too tight nor too loose, and fits like a capelet.

See? Easy! I used two different colors of markers (actually, rubber bands) for the beginning of the rounds. One color for the +8 round, and one for the knit rounds. I also wrote it out because I kept coming back to it. I just wrote:
+8 / K / K / +8 / K / K/ etc, and crossed each one off as I got done with the round.