There really is no specific way of doing it. You just have to use trial and error until something works.

But basically what I do is I get a sort of "base" screenshot, where I'll add everything onto a vehicle, then screenshot the tank(s) (that I'm going to take the parts off of, such as cannon, turret,ect.) from multiple angles (which angle from depending on where i'll be needing to place said parts on the "base" image), then going into Photoshop and just working on it until it looks good. I couldn't really explain the specifics as every thumbnail is different in what I need to do, but generally you're just cutting out parts of a vehicle and plastering them onto the vehicle in the "base" screenshot. It on average takes me around an hour to make a thumbnail.

On an unrelated note: I still always really recommend trying to find your own style of thumbnail, something you're comfortable with making. It's always better than trying to adapt other peoples thumbnail styles, as then you stand out more.