What is meant by "first preperation of trophies"?

Better you ask the person which used the expression.
Anyway, the trophy "preparation" consists of:
-skinning the head
-removing meat and eyes (personally I leave them attached to the skin)
-removing the tongue
-boiling the head (without tusks, if it is a pig)
-peroxiding the (clean) head

The expression "first preparation" could mean "skinning for taxydermy" or maybe the operation described above, stopped before peroxiding.