You need to have a secret keyring, with a secret key you know the password to, to be able to use this method.
To decrypt the file and verify the signature, you just need to decrypt the signed file using the PGPDecryptVerifyFile method.
You must specify what file to sign, the SourceFile.
You must specify what the signed file will be called, the DestinationFile
You must specify the Password of the secret key you are using.
You must specify the KeyFolder, where the secret keyring is located.
You must specify the KeyID of the public key you would like to use to encrypt the file.
You must specify the SignID of the secret key you would like to use to sign the encrypted file.