Just have to be as much as possible closer to the natural linking. Let's say you are writing interesting or/and important content. People are reading it. SOME people are linking it. For example 2% or 5%. If you're making link building that looks like these 2-5 % , then it's okay. But if you are going to make 50-60 %, then the anti-spam search engine protections will find it out and you will be black-listed.