I type several seed keywords into Ahrefs, then use filters to find keywords that meet ALL these requirements...
- keyword difficulty: between 0 and 3 (if I find a keyword with a KD higher than 3 I take a closer look at the other metrics to see if it justifies the effort)
- number of words: at least 5, but usually I pick keywords that are 7-8 words long
- number of backlinks of the websites ranking in top 10 of google: as many websites with 0 backlinks as possible. If you do your keyword research well you will oftentimes find keywords for which 8 or 9 of the websites ranking in top 10 have zero backlinks.
- keyword in title: if only a few of the websites ranking in top 10 have the keyword I'm interested in, in their tittle that's a good sign... providing that all other metrics are also decent and - more importantly - providing that most websites ranking in top 10 don't address the intent of the keyword very well. If the websites ranking in top 10 don't have the keyword in the title of their pages but the content of all those websites is extremely relevant to the intent of the keyword and address the topic of the keyword in detail I usually stay away from that keyword. But that's because I'm a big pansy and I only want keywords for which I can rank without effort and within a week if possible

... But if you're not like me and you're ok creating great content you might want to chase after that keyword
- topical relevancy of my competitors' websites.... if most, or all of my competitors have topical relevant sites I don't like it. I want to compete against general websites like wikipedia, quora, reddit, forums, facebook, pinterest, amazon, etc.... I don't want to compete against maniacs who work every day and churn content and backlinks 24/7 making it impossible for me, a simple and single dude having to do everything alone and manually. If I find other niche websites, created by other SEOers I usually stay the fuck away from that. Most experienced SEOers will tell you that that's a good sign because if you see other niche / affiliate sites ranking in top 10 for your keyword it means that you can break into that niche and rank for that keyword. Which is true, but it's a double-edged sword. If other SEOers like you can rank for that keyword it means that they will actively try to rank for that keyword, which means ongoing work you have to do to keep your spot. And this I don't like cause it puts pressure on me to keep working and... well, fuck working! I want easy, fast, delicious rankings all for myself, not to be forced to work everyday like a slave because morons with big budgets don't lift a finger but outsource everything to 3rd world workers while they just cash in.
And that's pretty much how I do it... or used to do it anyway, because today I saw that another google core update came on and apparently it shits on little guys like me even more. Today's update seems to favor (yet again) big budgeted sharks with high "authoritative" websites (read: websites with lots of powerful backlinks). So yeah, I'm not sure how much of my guide will still be valid after this update but you can try and see how it goes