You need an OTG USB cable in order to connect your android device with arduino.
Uses *.gcode files ONLY (ex: TEST.gcode)
Please read the online documentation :
1) GRBLDroid-USB will work only with GRBL 0.9+ versions
2) GRBLDroid-USB supports simple and advanced file mode.
I have followed the official instructions from here :
The simple mode is as described here :
Streaming Protocol: Simple Send-Response [Recommended for Grbl v0.9+]
And the advanced mode is as described here :
Streaming Protocol: Character-Counting [Recommended with Reservations]
3) GRBLDroid-USB checks if the commands are 80 lines long (including the last enter char)
4) It is recommended to purchase a dedicated tablet for this job with a DC IN 5V jack.
With this jack you can always recharge the tablet and at the same time it will stay connected on your CNC machine via micro USB OTG cable.
5)GRBLDroid-USB works for :
FTDI FT232R UART
CP210x UART Bridge
Official web page: http://www.trustfm.net/software/mobile/GRBLDroid.php
Online documentation: http://www.trustfm.net/software/mobile/GRBLDroid.php?page=Documentation