Link building is the most difficult task in SEO,Because it is time-consuming, and requires careful research, and the results of which are not guaranteed. At the same time I strongly believe that without link building your SEO campaign is not complete.For that,
Link building should not be a quick fix process.
Link Building should go hand in hand with other SEO efforts like onpage,offpage,social media etc..
Relevancy should be the key factor.