how do you write a blog?

Decide how often you are going to post. Some claim that posting at least once every day is best; Some also say that three quick posts a day are far more effective than one long post every three days. Others claim that when they update a blog every other day they get more readers than when updating two or three entries in a single day. Heck, write whatever you want – it's the content that matters. Whatever you do remember that for most bloggers, it's all about reading and many of them would prefer quality over quantity. Once you get started you'll find that you attract a certain readership, and you may have to adjust how you work your journal to appease and keep the readers you've obtained.

Um...you sit down and write something and then post it to Blogger or some other crap. Rational thinking, proper grammar, and knowledge of the world around you are not a requirement. A completely unwarranted sense of self-importance is often a major ingredient in any blog, especially if you get a few yes men in your comments.

Usually you have experience and you pick a topic you know a lot about.

It's also good to have sources where you get your information from.

You can earn a decent living if you get enough viewers, but not enough for a full on career