1) There are two extremes I have noticed when it comes to translating: one is to be too literal so that in the end the phrase doesn’t sound natural in the language it’s being translated. The other is to rephrase too much and add words from yourself so that the translated phrase no longer says what the original one said. So the idea is not to translate word by word, you need your translation to sound natural in the language you’re translating, but try to be as faithful as possible to the original as well.
2) One thing that is unique to translating subtitles is that you need to always have in mind the time the viewer has to read that line you’re translating. You may convey the idea in a very well-constructed, 20-words sentence, but if the viewer only has 1.6 seconds to read it, you lost them at half of it. And it might be frustrating. So the idea is not only to convey the message in the best, most comprehensible way, but also to find the shortest phrase that does that; of course, while still being faithful to the original.
3) Do not only translate what is written in a transcript/English subtitle, but listen to the video while you translate. Sometimes the transcript or the English subtitle can contain errors, and if you don’t listen to what the speaker says, you might end up perpetuating those errors in your translation. Also, hearing the accents, tonality, certain emphasis in the speaker’s voice, can be very helpful for the translator in how to translate those lines.
4) It would also be better to first watch the whole video in order to get an idea of what is being communicated as a whole. This helps you use a uniform language throughout your translation, or it can help you to better translate a certain phrase in the video, knowing some details revealed later on in that same video.
5) A small, yet pretty important detail is the punctuation marks. If it’s a question, place the question mark at the end; if it’s an exclamation, put the exclamation mark; and so on; so that even someone who watches the video on mute can better understand what’s being communicated.
6) The order of words can sometimes communicate a certain emphasis, so the translator should be careful with that as well. This goes hand in hand with what I said at point 3) about the emphasis in the speaker’s voice, and so on.
7) It’s good to watch the whole video with the translation on it, from beginning to end, after you finished it. In this way you can do a first-hand proofreading of your own translation. It’s also good to do it one day or at least a couple of hours later, so that your mind is fresh when you review the translation.