Start with something of broad interest first, rather than a niche. It's easier to find customers this way. If you have expertise in the niche, you likely know the fundamentals well. Teach those fundamentals.

Don't worry too much about production quality. The value is in the message. Many people fall in the trap of obsession on the production and get demotivated by the lack of progress. Just think of this as a recorded live talk. Prepare some slides, think about what you're going to say, and record.

The most difficult part is marketing. It helps to have an audience. People prefer learning from someone they feel they already know a little bit.