Building back links occurs in two ways ie, the manual way and the automated one. Using the manual option, one will create back links by manually posting the link of choice on the different blogs, forums, social media sites and others. On the other hand, the automated option can be done by paying for such links and or by automatically posting them using the available paid and free tools.
Personally, I use the manual way since it helps me locate and know where I have posted my link. This additionally helps me track such links so as to know whether they are still existing or not. On sites like Facebook and twitter, of course I use the automated option since my links will diectly be shared upon publishing a post.
If you search a keyword like "create back links" free from Google, trust me you will find lots of sites claiming that. You can try them and create some back links the automated way or simply go for manual blog commenting on other sites and etc.