What is the procedure to pollinate custard apple (sugar apple) plantation? I am having a plantation of Custard apple (also set as...

I am having a plantation of Custard apple (also set as Sugar apple)
The plantation is arround 4 yrs old and within is no sign of it bearing any fruits
It flowers though but the flowers wither bad and dosent get pollinated natuerally; So does that suggest we have to pollenate the flowers manually and if so HOW? please answer