Its really quite simple, if youve ever watched Blue Peter, make a badge and write CFI on it!
There is no definition of a CFI in the ANO or in JAR-FCL, except at an approved FTO for which see App 1 to JAR-FCL1.055 Para 15
http://www.jaa.nl/publications/jars/541268.pdf
For PPL instructing at a RF its whatever the management want to make it!