Basically following the instructions How to start

It clearly describes the steps in details:

• “Introduction into Android Development”
• “OpenCV for Android SDK”
• “Android development with OpenCV”

Several notes:

• Convert your project to a C/C++ project if you want to compile native code. See the section Building application native part from Eclipse (CDT Builder)
• Import the OpenCV Library and then reference the library in your project
• Configure NDKROOT and use it in C/C++ Build Command for build and Paths and Symbols for includes
• Configure OPENCVROOT and use it in Paths and Symbols for includes
• Change jni/Android.mk and jni/Application.mk to suit your needs