Dear Nicolas,
In that case, you want to edit the file "terms" of the view "checkout" via the menu Display>Views and add your checkboxes HTML there.
You'll also want to add there (or maybe in the step view file) some PHP to check that the checkboxes were checked.
That´s right so far but how to realize this concretely? Via Firebug, I can edit the page like I imagine but I can´t find the position where to add the html.
EDIT:
- I´m on the right path. Could you please tell me where the overrides PLEASE_ACCEPT_TERMS_BEFORE_FINISHING_ORDER and PLEASE_ACCEPT_TERMS are saved?