There are tons of threads on how to do that on bhw.
Use the search function.
You will finds guides even on google.
The process simplified is this:
Scrape tons of blogs (lets say tumblr). site:.tumblr.com keywordorabunchofrandomnumbers
Trim all urls. Delete duplicates.
Check dead urls.
Check metrics or check metrics after the next step.
Check the dead urls if they are available for registration.
Tools that will come handy are scrapebog and tumblingjazz.
P.S. at least for tumblr forget about getting niche related blogs. It is a myth except for niches like adult.