Rename the subtitle file exactly as the video file (e.g., Death_Note_-_01.mkv and Death_Note_-_01.ass ) in the same folder. Most media players (VLC, MPC-HC) will auto-load them.
What could improve
After hours of scouring dead forums, he found it on a site that looked like a relic from 2006: L-Files-Archive.net . The link was simple: Death_Note_Full_Series_EXTENDED_Subs.zip He clicked download. The Installation Death Note Anime Subtitles Download