For the simple solution you linked to, without the "modular slider" option mentioned, you need to have this kind of structure:
pages
|
05.sliderpage
|
- mypage.md
- slide_image1.jpg
- slide_image2.jpg
- slide_image3.jpg
Then you would put the first bit of Twig into your theme's templates folder as mypage.html.twig in this example. That Twig simply loops through every image file found in the same folder as the page using it, and selects them for the slider.
Also add the CSS listed to your theme's scss/_custom.scss (or create it). Unfortunately that example shows SCSS, which means you need to be running your CSS preprocessor for it to work.
There is also the lightslider plugin, which I find a bit clunky but much simpler to get going!
Let us know how you go.