The screws are fitted on the bottom of the spoiler(the spoiler already has holes for these) and on the both sides(before you fit the clip on side covers).I fitted 2 screws on each side.
I fitted mine years ago,But of I remember,I used 8mm head screws x 10 mm long.

The tricky bit to get perfect ,is the front!The sides need to be held tightly!I wouldnt rely on unstreched tape,you need to use a more unorthodox method and then leave it to glue overnight(using the sealant that mrbmwx5 suggested).
Leave you a pic of mine to give you a tip