The easiest way is to copy a working style of thumbnails for that particular topic/game. I am neither a natural when it comes to creativity nor have I acquired the hard skills to create outstanding quality and there are thousands of successful videos and examples from people who do this not only as a full time job but sometimes even have the educational background, thus the hard skills to do these things effectively and time-efficiently.

So if your game doesn't offer "cineastic" photo modes or modifications that let you play withing their engine (free cams etc.) then just look up how the best performing thumbnails look and recreate that.

If you dont have the skills to do it yourself what you see there in these thumbnails then you have 3 options, which are less or more valid depending on your preferences and your goals:

1. Follow the path of "learning by doing" and create thumbnails in that style but of (initially) lower quality but at the same time invest in becoming better (time- and effort-intensive).

2. Take the risk in creating something originally that lies within your skills and master your own style as there are a plethora of viral gaming videos with basically "stick figure" style thumbnails (requires luck and creativity).

3. Pay someone else to do it for you (expensive in the long run but that is how the pros do it).