Uhm, well, in a nutshell:
- build blog after blog after blog (look for niches, enough blogs about SEO)
- try to get high rankings in SE's (it takes some skill, you have to know how to write, use keyword rich content, etc.)
- get many visitors (shouldn't be very hard: good content will get indexed and will yield visitors; the more content, the more visitors, hence blog after blog after blog)
- 1% or 2% of those visitors will click your AdSense ads (just an example but it's realistic)
- earn money (the more blogs, the more money)

;-)