I feel incredibly unqualified to give advice, as I'm only working on my 3rd video now and it's taking far longer than I wanted it to, but how I did it? First, record gameplay. A full playthrough of what you're covering, if need be, because while writing you can refer back to it. Second...

...write script. This was the hardest part for me, the ADHD or executive dysfunction made it a constantly uphill struggle. But keep chipping away at it. Even if I couldn't concentrate, just write something, a note, bullet points to flesh out later on, anything, just keep making tiny steps.