What are the things that I should prepare for this session, in terms
of order of dress, language, content?
In an actual court room, suit and tie is preferred for men and formal dress for women for lawyers and professionals.
Your "Sunday best" or what you would wear to work on a typical day would be appropriate if you don't have a suit and tie or the equivalent.
Avoid nightclub looks, any garment or ornament with a message, or more than minimal jewelry or accessories. For a video testimony, solids come across better than complex patterns if the video resolution turns out to be low. Your clothing should not be a message other than that you are a respectful reputable member of the community.
Language and Content
You should avoid colloquialism or profanity (except in a quotation from someone else about what they said out of court), but should not try to affect an accent or dialect that you cannot speak naturally. If you grew up in rural West Virginia, don't try to pretend you grew up on the Harvard University campus.
If you need an interpreter, you should let someone know in advance as this can be arranged.
Generally, you should only speak in response to question, and should leave a "beat" between being asked and answering any question in case someone says "I object" before you start to answer.
You should listen to the question you are asked, truthfully answer the question you were actually asked (not necessarily the one that was intended to be asked), and stop when you have answered it. (This is harder than it seems.) If asked an open ended question, take whatever time it takes to answer it completely.
It is appropriate to instead ask for clarification of a question if you couldn't hear it properly or if you don't understand it because it uses a word you don't understand.
If the question assumes something that isn't true, correct that wrong assumption. For example: "what did you see the defendant doing at the football game on Sunday?" "I didn't see the defendant doing anything at the football game on Sunday because he was having dinner with me after the baseball game on Sunday."
Don't rush. Most people talk fast when they are nervous or try to over summarize. Everyone has gathered at this moment, in part, to hear what you have to say. Take the time necessary to say what you need to say to tell the whole story in response to the questions you are asked without summarizing the facts.
A little emotion can be appropriate in your tone, but stay civil and don't blow your top. As a rule of thumb the person who seems most reasonable usually wins.
The judge can be referred to as "your honor". Lawyers can be referred to as "sir" or "ma'm".
In content, you will be normally asked to provide your opinion on things like "is there any reason he would be a flight risk" (ideally answering with not just a "no" but a "he owns a home and has deep ties to the community and would never flee."), "is he a threat to the public?", "does he have a good reputation in the community?" ("Yes, I'd trust him with my money and my children.").
Don't make things up, but focus on facts that reasonably demonstrate that he is unlikely to flee because he has too much to lose, firm ties, and no place to go.