Think about your niche. What words describe that. Look up those things on YouTube and see what autofill looks in. Look on TubeBuddy's keyword search and search those words to see what the score is.
My niche is Canva & how to use it for YouTube. So then I use things like Canva Tutorials, Canva, how to use canva for youtube, learn canva for youtube, The Creator Classroom (channel name should always be featured in keywords), and more. I currently have 335 characters, but have room for 500 so I'lll be looking at my keywords next week to see if I can adjust any or add any to my channel.