It is always easy for me. Just as iaoish says: keep the video file and the subtitle file in the same folder, then open the video with VLC Player. It helps to name the subtitle file the same as the video. For instance: Oldboy.avi and Oldboy.srt
If you do this, the subtitles should load automatically. If you're burning a dvd and, say, you're using convertxtodvd, then having the subtitle file in the same folder as the video will automatically add the subtitles to your dvd, in which case all you'd do is press the subtitle button on your dvd remote to turn them on. Or, you can double click the subtitle drop menu for the movie in convertxtodvd and select "default," which makes the subtitles automatically play on your dvd without needing to press a button.