I do not know the shape of your poncho but I guess it was knit as a flat blanket type thing and now needs to be sewn together.
You can probably find a sketch for that with your pattern or there is a photo of it?
So I guess you can figure out how to assemble the poncho.
Now comes the question “HOW do I sew?”
Well, first you bind off and you finish all other things (except maybe the weaving in). Then you get ready to sew.
I recommend [B]matress stitch[/B]. There are instructions here in the video section and there are multiple good videos on Youtube.com
you use a darning needle and the same yarn you have been knitting with. You work from the right side and therefore can line up the pattern really well (if needed in detail). The videos will show you how to join the pieces in the actual technique. When you do it right, there is no stitch visible. Just a little “fold line” where the pieces go together and a little ridge on the inside, but relatively flat.