How can I publish the slider, where I want it to be? (WordPress)

Usually WordPress themes aren't giving a good widget position to put something right under your menu, or in other places where a slider would be nice, so the way you can do that is you could use our php code publishing.

To figure out which codepart you would need, you should check out this video.
The part where people mostly want to have their sliders is right under the </header> code. That is usually in the header.php file, but it could be in other files too. You can search for a codepart in multiple files, as you see  here from step 5.
So all in all, you just need to find the codepart between your theme's files, and you can put your slider there!