Using dividers in menu navigation – where to add code?

This is one of the code i have used in the past. The below code goes in your functions.php

class MP_Footer_Menu extends Walker_Nav_Menu {

    function start_el(&$output, $item, $depth = 0, $args = array(), $id = 0) {
        STATIC $counter = 0;
        if ($counter++)
            $output .= '<li class="divider">/</li>';
        parent::start_el($output, $item, $depth, $args, $id);
    }

}

This is an extra parameter passed in the array passed to wp_nav_menu. It should be present inside your php template file (probably header.php or footer.php)
'walker' => new MP_Footer_Menu()

If you can’t figure out the exact place to put this line, you can do a folder-wide search for wp_nav_menu in your theme & then update your question copy-pasting some nearby code & then someone here will tell you the exact place.

This should get the divider in place but you’ll most probably also need some CSS. I suggest that once you’re done with the above & if you have some problem in CSS, go over to https://stackoverflow.com/ & ask that question separately. You’ll get better answers there for CSS.