Making a list remains hard. And up until a few years ago, doing your keyword research was much easier. You could simply check Google Analytics to see which terms people used to find your website. That is no longer possible. So you’re pretty much left in the dark about the terms people use in search engines to end up at your website. Luckily, there are some other tools which can make your keyword research a bit easier, and could help you speed up the process a bit:

Note: If you can’t justify using Ahrefs on a monthly basis, you could sign up for a month, do your KW research and cancel. That said, if you can justify the monthly pricing it’s well worth keeping because you’ll get access to the ongoing functionality such as rank tracking and web monitoring. It also means there’s no need to use any other tools to track rankings or monitor mentions on the web.
Once I have a list of phrases, rankings, and volumes from these tools, I'll look to internal tools (maybe Excel, Access, or another database) to organize, classify, and forecast opportunity. This is where I'll estimate a competitor's traffic based on volume & position CTR, set goals for a target position, and estimate traffic based off that position's CTR and keyword volume.
Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website.  In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC).  I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.