step 1: collect winning samples.
go on youtube and find 5-10 example thumbnails YOU LOVE and are performing well for that creator.
hire someone on fiverr to make your thumbnails so they look/feel exactly like the samples you provide them.
step 3: rinse & repeat.
keep looking out for great thumbnails and keep working with your thumbnail designer to help make them better.
pro thumbnail tip #1: no more than 4 words per thumbnail!
pro thumbnail tip #2: don't use the same words that are already in your title
pro thumbnail tip #3: get 2-3 thumbnails per video and split test them using YouTube's split test feature (see below for the test feature image)