My scripting starts in my head with how I'm going to present certain parts. If I have dialogue for what I want to say, I will jot it down as such. I will write down main points I'm going to cover, but in no particular order. The point is just try to get everything you want to cover.
After I have what I believe is everything, I order those points how I think they should be. Stuff plays through my head, and I'll write some dialogue down if it comes up.
Even with a script, I improv most of it. The reason is that I don't want it to sound scripted, and it's also more challenging. I myself don't like videos that sound scripted, and like stuff that sounds more natural. The main point of my script is to make sure I cover everything.