The answer to your specific problem is to use cmake-gui which provides you a visual interface to ease the process of setting up variables, like your ffmpeg package location.
However I have 2 recommendations:
- Download the most updated toolchain.
- DO NOT USE THIS GUIDE. Use instead the toolchain that you find in the package in sources\platforms\android\android.toolchain.cmake: read inside, you'll find the correct instructions at the beginning of the file.
Another optional recommendation: use ninja as build system, much faster.