There are several ways to become such links. Some of the way is commenting on comment luv permitted blogs. Comment Luv permitted blogs usually suggestion free do follow backlinks. Several such blogs still gives nofollow backlink, which is of no use. There is still a method by which you can identify whether the backlink is do follow or no follow. It is good if your site is receiving a do follow link since a high PR website as it will pass extra link juice associated to low PR website.