Please install the following prerequisites (instructions for each follows):
- Android Software Development Kit or SDK (for Java code): http://developer.android.com/sdk/index.html (Android Studio)
- Android Native Development Kit or NDK (for C/C++ code) http://developer.android.com/sdk/ndk/index.html (This can be installed through Android Studio's SDK Manager)
Now build the Android app using Android Studio/gradle
This will produce an unsigned Orbot package APK.
To produce a usable package, you'll need to sign the .apk. The basics on signing can be found on the Android developer site:
http://developer.android.com/guide/publishing/app-signing.html