Google is also against manually creating links back to your website except in situations where it is obvious to do so. For example, if you have social media accounts at places like Facebook, Twitter, etc. then Google expects you to tell people what your website URL is in your profile. So that is a good place to manually place a link.
However, if you go around publishing articles with links back to your own site or if you go dropping links on forums or if you go create links using blog comments, those are all examples of things that Google has publicly stated they do not approve of. Those are not natural links.
You can get away with manually creating links, but if you are going to do that, and it is a website that you really care about, then at least try to make it look legit. Like if you are on a forum and there is a discussion happening that relates to something you wrote a blog post about, then go ahead and link to your blog post from inside your response on the forum. Can you see how that looks more natural?
Don't just go around to various websites dropping links from wherever you can. If you do that, you are increasing your risk of a Google Penguin penalty. You can get your site penalized with just a few dozen of those low quality links that get dropped at random websites. And if you are going to do any of that, make sure you link to deep pages on your website. Spread the links around a lot. Don't just link to your homepage. That looks really bad. And also make sure you use a whole bunch of different anchor text. If you are manually creating a link, don't repeat the same anchor text. Change it every time.