UPDATE: Go here for a quick tutorial and a better way on how to insert adsense and other contextual ads.
Looking for a quick way on how to manually insert Google adsense code on your blog posts?
Placing your adsense code within your blog posts entails a time-consuming process of editing your site’s theme and the underlying PHP files. If you really want to do it this way make sure that you have basic knowledge and understanding on how to edit PHP and CSS files. These are the web programs that control the display and appearance of your website.