The question "How [do I] write in a [Android HCE] emulated card?" does not really match the concept of Android HCE.
With Android HCE you do not emulate a memory card, hence there is no such thing as writing to/reading from such an emulated card by default. Instead, you emulate an APDU based smartcard application (note: application, not memory area, not whole card). APDUs are commands that you can send to a card (for instance with a PC/SC-compliant contactless smartcard reader). These commands are then interpreted by your emulated smartcard application (i.e. your HCE service) and used to generate response messages.
So, to summarize, you define the commands (based on ISO 7816-4 APDU structure) that your application should understand and the response message format (again based on ISO 7816-4 APDU structure) that your application sends back.