Probably have been devoting most of my energy into professional looking thumbnails than video editing actually, Even made some process videos.
Usually I'll find a background with some type of texture on it and saturate it for every new video. I'll Photoshop an image from/related to the video with a white outline around it and a drop shadow behind the image. Angled text seems to work really well too!
Small piece of advice from me for your channel art: be aware of the trends! Trends are moving quite fast, you should change your art once or twice a year. Keep it trendy and people will be more incline to click
I'm a graphics designer and I'm new to the forum! As a little gift, I've made a professional YouTube banner for you all to use! The link to my shop can be found in the description of the video with instructions on how to download the file. On my store use the code "banner4you" to get the banner for free.
Banner download in the description:
Let me know if you like the design and if you're able to edit the banner properly. Let me know if you have any feedback... I'm happy to hear!