Toner is essential for neutralizing unwanted brassy or yellow shades after bleaching. It's best to apply the toner right after bleaching or on damp, towel-dried hair. Follow the instructions provided on the toner's packaging, which usually involve mixing the toner with a developer and leaving it on your hair for a specific amount of time. Be sure to check the progress of the toning process periodically to avoid over-toning, which can result in an undesired hue.