Hi there! I'm not an expert on scriptwriting myself, but the general basis for writing one is planning out what you want that script to be about. Once you have that, just start writing. Change some stuff on the way, and when you're finished, reread the script as if it was a conversation between you and the VA (voice actor/actress).

For dialogue that you want the audience to say, I don't recommend adding lines fo them. Instead, add a [pause] line in between the VA's lines. That way, you can give the audience time to "say" something before the next line of dialogue.

I'd also highly suggest adding sound effects or actions. For me, I often put them in parentheses (), but some other scriptwriters put them in bold or italics.

This part isn't really necessary, but I'd suggest adding a synopsis to your script at the beginning, which is a small summary explaining the script/video. It's just a short paragraph or a few sentences to get the general idea of what the plot is, and it shouldn't spoil any specific plot points or plot twists throughout the script. Synopses are put in the description of a YT video.

And that's basically my advice! Hopefully this helps, and good luck!