Set up a website where people can send messages to themselves in the past in multiples of ten seconds, for a cost. Program it to automatically increase the cost as you start running out of bandwidth. Let other people figure out what to do with it.

There are a few experiments that you should try to see what you could do. For example, it seems like a good idea to have it send a message about a car accident far enough back to prevent it. But if you get the message that your car will crash, you'd have to not drive and send the message to prevent a time paradox, which means that you might get the message even if the car didn't crash. You could experiment by using things like coin flips to determine car crashes. My guess is that if you send a very specific signal in the case of something bad, then you're very unlikely to get that signal unless it would happen. Otherwise, every signal would be self-consistent, and thus equally likely.