By not providing "FindOpenCV.cma

The error you're seeing is that CMake cannot find a FindOpenCV.cmake file, because cmake doesn't include one out of the box. Therefore you need to find one and put it where cmake can find it:

You can find a good start here. If you're feeling adventurous you can also write your own.

Then add it somewhere in your project and adjust CMAKE_MODULE_PATH so that cmake can find it.

e.g., if you have

CMakeLists.txt
cmake-modules/FindOpenCV.cmake

Then you should do a

set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${CMAKE_CURRENT_SOURCE_DIR}/cmake-modules)

In your CMakeLists.txt file before you do a find_package(OpenCV)