With my round fabric it makes no difference which is the front or back so I can just start pinning my collar.

Starting on either end, you want your collar to have half it's fabric on one side of your poncho then folded over your neck hole edge and pinned down to the other side.

Keep folding and pinning your collar to your poncho until you've gone all the way around and your collar ends meet. Over lap ends and sew along entire collar and once along the edge where your fabrics overlap.

With your rectangular poncho it does matter what is your front and back (you want your collar hem to be in the back and not randomly over a shoulder) so the only difference is where you start.

Laying your poncho in front of you, grab your collar. You want to start pinning the middle of your collar to the front of your poncho working your way back. Continue pinning your collar until you reach the back.

cut any extra fabric and overlap your ends. Sew along the entire collar and once more where your fabrics overlap.