Over the years I have gotten better and better at making thumbnails, and it is still one of my favourite parts of the process. Every one starts with research - digging for outliers in the niche and cross-referencing what is actually working - then narrowing down, mocking up, and testing. These days concepts usually run through A/B (and sometimes C) spread tests, slight variations of the same idea competing against each other, with a completely fresh concept a few weeks later if the data asks for it.

Every thumbnail below is shown before and after - drag the slider to compare.