I think the conventional form for this construct should be: "How to choose your mirror manually"
It is true that in recent times the resistance to splitting an infinitive (by inserting an adverb between the "to" and the verb) has been weakened by what used to be thought of as incorrect- and what is now given a free-pass because it is common- usage. Nevertheless, the "correct" construction cannot be criticised and will be generally accepted as "correct" by all, even those who don't know the difference:-)
