Cast on circular needles enough stitches (divisible by 4) to fit comfortably over your head.
Round 1) Place that different marker to mark the beginning/end of the
round. Knit one quarter of the stitches, *place a marker, k another quarter
of the stitches*. Repeat between * until the
first marker.
Round 2) Purl all.
Round 3) *Slip marker, yo, k1, yo, k to next marker*. Repeat between markers to end of round.
Repeat rounds 2 and 3 until you are fed up, run out of yarn or have the length that pleases you. Bind off LOOSELY or add a knitted-on border such as is done on some shawls.
A collar is an optional add-on by picking up stitches at the neck. I did a 2 x 2 ribbed one on the poncho for my daughter. None on mine because there wasn't any more yarn!
WARNING: your final stitch count may be astronomical! If you haven't either a very long circular needle or several of the same needle size or a NeedleMaster set, think about getting one.
Yes, the yo at the beginning/end of the round will be visibly bigger than the others. Only a knitter might notice. It beats sewing a seam! I haven't figured out a cure.
Yes, using circulars is supposed to eliminate the need for purling ... but how else to make garter stitch on circular needles? I hate seaming more than purling!
One knitting book I read some time ago described garter stitch fabric as akin to velvet. I agree! I love how drapey it is, how forgiving of uneven tension, how super cuddly it feels. It must NOT be blocked; no matter what fiber is used. Mine is my travel buddy; now a pillow, now a lap-robe; and, of course, a poncho.
Vary the number of stitches between the yo's. Use wild variegates regardless of the wearer's body shape/size - the stripes are all diagonal. The resulting poncho should be a flat square with a hole in the center. It will probably be worn with a point front and back and on each elbow. Works well under a back-pack, too.
Enjoy! I have never seen this pattern in any publication. If anyone else has published it somewhere, I apologize. I "unvented" it myself some 20 years ago and freely share it.