watch the shadow of your drop plane.   The shadow is the drop point.

Drop by spamming the   .  (Period) key same as launching torps from TBs.

There is a slight delay before the paratroops deploy so spam a short distance before the drop point.

I've found that for the small islands, if you start spamming just as your planes shadow
passes the edge of the island, they will drop pretty close to the center.    You have a little more room on the island base.

The big base that is on the edge is a little tricky in that if you let your drop plane get too
close to the border, it will automatically get shot down.

Immediately after drop, start your drop plane back to your ship.   Otherwise, it will circle
and, if near the border, go down by auto shoot down.    Besides, you don't want it to be fighter bait.