1. Pick a topic I'm interested in, or that I would like to be interested in and follow regularly. It could be anything from sci-fi to front-end web dev. My only requirement would be that I know where to find other people who are interested in that topic, and that they're a group that's comfortable with email.
2. Start a regular newsletter where I'd aggregate content around that topic and maybe editorialize a little bit by adding my own summaries. I'd keep it short and simple and send it out 3-5x a week.
3. I'd throw up a simple landing page where people can subscribe to the newsletter, then advertise it by sharing my insights on Twitter and in various online communities every now and then. If this is your only focus, you should be able to sign up at least 250-500 ppl/mo. If you can't get that, then your content probably isn't compelling.
4. Sell ad spots in the newsletter to advertisers, specifically advertisers who are in the same niche that your newsletter is. You should be able to make at least $50-100 per 1000 subscribers for each email you send out, which is one of two reasons to send frequently. (The other is that high frequency helps people become familiar with you and turn into regular readers.)
Ideally it's a win-win-win. You get motivation to keep yourself up-to-date with a topic that you care about, and you also make money. Your advertisers make sales. (Hopefully they're good companies that you hand-picked.) And your readers get free high quality info on a topic they care about without having to do the research themselves.