I used to implement AdSense code manually. After a year I found a good plugin for inserting AdSense code in WordPress.
Ad Inserter: This is a very good plugin to insert AdSense script anywhere in your website. You really need to be a coder to place the code in sections of WordPress. The plugin take care of everything.
Even if your website doesn’t have any option to add adsense code in header or in-article, you can insert the code using Ad inserter plugin. The plugin is almost free. You have to pay for extra feature. Even the free version of this plugin is more than enough. You would never even think to pay for premium.