Creating thumbnails is quite easy for me but I'm learning everyday too. I try to create it before I finish editing my pictures. I use Lightroom to edit my pictures then I use Canva to create the thumbnail I'll post. On canva, you can type in youtube thumbnail layout and it shows you a couple. Just replace the pictures there with yours. There have been times I've redone my thumbnail after posting a video though. I always want to feel comfortable with the pictures I post so that's that. I hope this was helpful.