there are small screws into the plastic from behind the dash. I bought a set years ago and some holes in plastic did not line up with the holes in the dash, left & right. slotted the dash holes a bit. can be a pain getting screws started.
seeing as the screws go into plastic they are like sheet metal screws ,short and small, I believe. it's been 25 yrs or so. what was in your original plastic pieces. As I remember, I used the original screws and SLOWLY worked them into the holes of the new plastic before installation. I do know it was a pain holding the screws and getting a driver on them!