This will be only a research, don't build any links yet! Research and make a list of related authoritative sites/pages (forums, blogs, Q/A sites etc) where we can get do-follow link. Sites must be completely un-spammed, with DA30+. I'm not interested in sites where you can quickly register and add a link, these won't usually work. After you have found a relevant site, next step is to find a way to get natural looking link. Put everything to excel: site + method of getting a link. Don't build any actual links yet!