Once you have classified your data by risk level, you need to apply security measures to protect it from unauthorized access, modification, or destruction. These measures may include encryption, which transforms data into an unreadable format that can only be decrypted with a key; authentication, which verifies the identity of the users or systems that access your data; authorization, which grants or denies permissions to the users or systems that access your data; auditing, which records and monitors activities and events related to your data; and backup and recovery, which creates and restores copies of your data in case of loss, corruption, or disaster. You should apply security measures according to the risk level of your data - for example, high-risk data may require stronger encryption, stricter authentication and authorization, more frequent auditing, and more reliable backup and recovery than low-risk data.