Q. If I already have some schach on top of the suka, but not all of it has been placed, can I already start hanging decorations that are not kosher for schach (plastic flowers and paper chains)

A. Horav Shlomo Miller’s Shlit’a opinion is that the decorations have to be positioned only after the schach is already in place, since they become “batel” or annulled to the existing schach and therefore one is permitted to sit under them.
He pointed out that once you have schach enough to make the Suka kosher, even if some areas are not properly covered yet, you can already place the decorations on all areas.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a.