first, i make a befunky collage and remove the borders so i'm not restricted to them. i get lots of pink, glittery images and screenshot my face from a moment in the video that was memorable. i layer the images into an aesthetically pleasing collage and then add some glitter to it. finally, i add a large piece of text with the video's name.
Program wise for editing them, if you're basically broke af like me you can always use Picmonkey! It's a website that lets you do everything from collages (which some people suggested you should do as a TN) to just editing pictures! It's basically a very poor mans editing site which, if you mess around with it, can have some great results!
So I went ahead and took a look around your channel. I'll give you some feedback on your thumbnails on what I think what works and what doesn't, now of course this will only be my opinion
So for the undertale series, I am absolutely not a fan of the layer style nor the font of the text, it is very hard to read. I would recommend you switch to a clearer font and go for a solid color with perhaps a light gradient on it if you wanna be fancy, but even if you just change the font and make the text solid white it should be easier to read. Also, I feel like having your name taking up a whole band on the left side is quite unnecessary, your thumbnail should be recognizable by design and not by having your name on it, now maybe if you have a logo throw that on it. Also, the layout of your thumbnail is inconsistent, for Part 1, the words "Part 1" are in the bottom left corner but for part 2 they're centered and for part 3 it's in the top right corner. Also some of the parts have special titles others don't. Also I would recommend that if you give a special title to one of the parts you could make the little pictures on the thumbnail relate to it, not necessarily with pictures from undertale, just little cartoons to illustrate your point on some undertale background so that people still know it's undertale.
For the other thumbnails, I have more or less the same opinioin. I'm definitely not a fan of the "Lizzeelive" text taking up all that place. And your thumbnail layout is quite inconsistent throughout your videos. Also once again, if you decide to give a special title, like what you deem important enough to put in caps in your titles, I would suggest summarizing it in 1 to 3 words and writing that very visibly on the thumbnail, because text will definitely be more eye catching. Don't write too many words on the thumbnail otherwise each word is smaller and therefore won't grab as much attention and your thumbnail just loses impact overall.
One little tip I could give you to help out would to maybe slight blur out the background behind a text on a thumbnail to make the text pop out more. Also, if I were you, I would try working out a color scheme throughout your thumbnails to give them a little style that would be personal to you, additionally to an identical layout for each thumbnail