Partly it's about building habits around them. I watch out for any time I have to spend half an hour or more figuring something out - that's generally a sign that it's worth tidying up my notes into a TIL, since the internet is clearly missing that piece of information!

I keep very detailed notes on everything I'm doing already - either as a VS Code scratch document or Apple Notes or often a GitHub Issues thread. A lot of my TILs start by me pasting those notes into a Markdown file and tidying them up.