I added after
get_header(); ?>
<aside id="secondary" class="left-sidebar widget-area" role="complementary">
<?php dynamic_sidebar( 'sleft' ); ?>
</aside><!-- .sidebar .widget-area -->
Then
aside#secondary {
width: 20%;
float: left;
margin-right: 15px;
}
Now… if you’re going to have both sidebars, you’ll have to adjust the
@media screen and (min-width: 61.5625em)
body:not(.search-results) article:not(.type-page) .entry-content {
float: left;
width: 50%;
}
or something like that. You can customize the CSS as you need, but this should put you on the right track.
Also.. FWIW, I created a child theme, and added this to it, rather than modifying the twentysixteen theme directly. This is the recommended method.