Dialogue.
Placing Put each piece of speech on a separate line or lines and place it underneath the relevant speaker. You may have to edit more to ensure that the lines are short enough to look placed. Try to make sure that pieces of speech placed right and left are "joined at the hip" if possible, so that the eye does not have to leap from one side of the screen to the other.
Dashes. Put each piece of speech on a separate line and insert a white dash (not a hyphen) before each piece of speech, thereby clearly distinguishing different speakers' lines. The dashes should be aligned so that they are proud of the text. The longest line should be centred on the screen, with the shorter line/lines left-aligned with it (not centred). If one of the lines is long, inevitably all the text will be towards the left of the screen, but generally the aim is to keep the lines in the centre of the screen. Note that dashes only work as a clear indication of speakers when each speaker is in a separate consecutive shot.
Extracted from bbc.co.uk Online Subtitling Editorial Guidelines V1.1
ellipsis [...] or three periods, three full-stops or three dots. Used in subtitling when a subtitle extends over two separate shots or to mark the actor's hesitation in speech. Subtitlers can sometimes use ellipsis somewhat indiscriminately, which can detract from the content of the ST. Technically, dots take-up valuable character space. "With long speeches, break the line at the breath points, according to the actor's natural pacing."D. Bannon 2010: 109
"Ellipsis is a term for three or more dots separated by a space each to indicate missing text: ". . ." Prefer period dots without spaces: "..." D. Bannon 2010: 108
Length. A subtitle's length should normally be between 37-40 characters, including spaces and punctuation. (see http://bbc.github.io/subtitle-guidelines/#Line-breaks)
Lyrics. Translate lyrics in italics when they are relevant to the scene only.
Poetry. If a character is reciting an author's poem then indicate this with quotation marks. No need if the poem is their own.
spotting Synonyms: timing, cueing. Formerly, a mechanical procedure carried out independently of the content and nuances of the dialogue by technicians (repérage). Now used to describe the process of defining the in and out times of individual subtitles. (Glossary of subtitling terminology)
Timing. Subtitles should be between 1-6 seconds in length. (http://bbc.github.io/subtitle-guidelines/#Timing)
Voice-overs. Do not italicize if the voiceover is a constant feature of the clip. If it indicates an internal monologue within a conversation then italicize. If the character is meandering in his thoughts alone, you do not need to italicize.
Written (on-screen) information. A variety of formats are used: MAIN STREET - <Man Wins Lottery> - [The Old Man and the Sea] - (Ella's House of Jazz)