Are you sure it’s correct that "approach to" should be followed by “doing” and not “do”? Who told you that”? I assume you are just using the verb “do” as an example, but are talking about any verb that might follow “approach to”. Is that correct? In the following examples ( from https://en.oxforddictionaries.com/definition/approach ) the verb following “approach to” is not using the -ing form: ‘Load management is a systematic approach to control the operation of electrical equipment.’ ‘The design and methods are a reasonable approach to study the association between modifiable risk factors and injury from car crashes.’