place the psg spoiler onto the car with the sxi one still attached, make sure it is in the centre and mark round the legs with a pencil.
then take off the sxi spoiler and drill 2 holes inside were you marked the legs.
then once you have drilled the 2 holes all you need to do is drill 2 pilot holes in each of the spoiler legs by drilling straight through the sxi spoiler. so the holes match up.
now get some 3/4" screws m6 or m8 i cant remember which i used.
once you have attached the spoiler to the original one, make sure it goes back onto the car.
unscrew the spoiler from the sxi one and sand the sxi spoiler down with wet and dry. sand the bottom of the legs down with sand paper to get a rough finish and you a bonding agent.. and bond the spoiler to the sxi one.
once its all dried sand the excess bonding agent from the legs of the spoiler and use body filler to smooth it in. use wet and dry to get an excellent finish and then take it to a body shop to get it painted.
sorry for the long reply.,
im pissed and tired so it probably didnt make alot of sence.
if you still need help post it and i will get you pics.