Physical notes is an awesome start, @Damon nailed that. Another good practice is to go to the video of creators whom you emulate, those who are similar to our own channel and look into the comments of their videos. See what people are asking for.
It helps to have an inquisitive nature. Sometimes, ideas are born out of writing out questions potential viewers might have. Or if there is something "cool and interesting" to show and tell about.
I type in my ideas from my phone or computer into Google Keep where I keep various notes. When I get a hot idea, I gotta capture it. Unfortunately, my suggestion is no good when I am in the shower and I get a good idea! LOL!