When I start working on a ticket I actually start writing up a PR early on.

As I figure out my manual testing, I'll write out the steps that I took in my PR.

I've found that writing it out as I go does two things: 1) It makes it easier to have a detailed PR and 2) it acts as a form of rubber-ducking. As I'm updating my PR I'll realize steps I've missed in my testing.

Something that also helped out with my manual testing skill was working in a place that had ZERO automated testing. Every PR required a detailed testing plan that you did and that your reviewer could re-create.