You need to have a good photo editing software first, I suggest you to use “paint.net” if you don’t have it yet ( Paint.NET - Download ). Next, there’s a website which you can find icons for your thumbnail called “flaticon” ( https://www.flaticon.com/). So you need to find the icon you want, then download it. After that, import it to paint.net, add the words and backgrounds then you’re done.
Well if it’s too complicated then I can’t help you…