If the S1 is a full face then very very easy.
Is the lining on the left side of the helmet secured to the rim like on the C2?
If so, you'll need to easy it apart and then the can get the Scala bracket to fit and get access to the two tiny Allen screws that clamp it tight. Otherwise it a simple job to squeeze the bracket twixt liner and rim/shell of helmet.
Then it is a matter of feeding the speaker(s) to the ear recesses.
Job done.