|Tuck the poly over the window sill and|
under the window frame.
One of the tricks Matt showed me is how to seal the poly vapour barrier to the outside windows. Basically, you blow spray foam around the frame and then tuck the poly into the spray foam as it expands out and to the window frame, just below the drywall trim. The joint is then sticky-taped for good measure. The corners will be a bit of a weak spot, but considering all the other places in the house I've wondered about, it should be fine.
|Allow the foam to expand over the poly, pressing it|
tight to the drywall trim.