There are so many sliders out there...
each one has to be added in a different way.
Mainly it works like this:
you need to load the jQuery slider CSS, e.g.
<link href="jqueryslider.css" rel="stylesheet">
Then you need to load jQuery library (if it's not added to your HTML yet) and the jQuery slider code, e.g.
<script src="jquery.js"></script>
<script src="yourjqueryslider.js"></script>
Now you need to add the HTML markup from the jQuery slider to your HTML...
Nearly any slider has instructions
a. on their website
b. in a README file
Good luck 