Assuming you do not have easy (ie local) access to your direct manager, then the best way is to do it through a phone conversation (brief, by the way) followed up with an email to your direct manager and others if he/she directs you to do so. Keep it short and polite. Before that, be sure you have accessed any records (ie sales accomplishments, kudos, etc.) that would help you in future endeavors (that are not protected as confidential business information) because you should expect to lose access to those sources relatively quickly.
