Timing: Captions should be timed to the audio, or if necessary within 3 frames of the audio. Understanding how to time dialogue before/after a shot change is key to creating perfectly timed captions.
Appearance: Line breaks are super important, so don’ forget to apply all the line break rules. It is better to segment a long single-line caption into a two-line caption, distributing the words on each line. This is because the eye and the brain of the viewers render a two-line caption as more bulky and, as a result, accelerate the reading process.
Spelling and grammar: Be sure to know the spelling and punctuation conventions of the language that you are captioning. Always run a spell and grammar check. This 5 minute task can be the difference between a good and bad file.
SFX: The aim of captioning is to give the information that a hearing person receives by listening. Not only is information provided through dialogue but many other sounds, or even a lack of sound, may also be vital to the understanding of content, context or plot. All of these must therefore be conveyed through captions.
Formatting: Captions should be positioned at the lower part of the screen, so that they cover an area usually occupied by image action which is of lesser importance to the target film. That being said, raise captions that cover on-screen text.