Question About local signing API

I want to use local signing API but i cannot understand.

How can i use local signing API?

I want to put parameters like privatekey,publickey,long ID for "manually", not automatically.

Someone can help with examples?

Follow the constructor nxt.blockchain.ChildTransactionImpl#ChildTransactionImpl which builds and signs a transaction. That constructor takes all the different pieces of the transaction and assigns them to class fields. Then it computes the signature using the private key, i.e. sign the transaction. That happens at line #171:

this.signature = Crypto.sign(bytes(), privateKey);

If you follow the bytes() method you will see how the transaction is converted to a byte array.

What is fxtTransaction?