I have already shared a detailed information for creating child theme with Underscore starter theme. In this post, i will tell you how do I create custom template for Category Archive page with Underscore child theme

STEP 1 : Go to wp-content > themes > (your current theme). You should see a file there named archive.php.

Open it and find the below line

get_template_part( 'template-parts/content', get_post_type() );

Replace with

get_template_part( 'template-parts/content-archive', get_post_type() );

STEP 2 : Copy-paste the below code in text edtior and save it as content-archive.php in template-parts folder (col-md* class will only work if you have added bootstrap)

<?php
/**
 * Template part for displaying posts
 *
 * @link https://developer.wordpress.org/themes/basics/template-hierarchy/
 *
 * @package new
 */

?>

<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
<div class="entry-content">
	<div class="col-md-3">
	<a href="<?php the_permalink(); ?>"><?php if ( function_exists("has_post_thumbnail") && has_post_thumbnail() ) { the_post_thumbnail(array(300,260), array("class" => "img-responsive home-thumb")); } ?></a>
	</div>

	<div class="<?php echo (has_post_thumbnail())?'col-md-9':'col-md-12'?>">
		<h2 class="entry-title"><a href="<?php the_permalink(); ?>" title="<?php echo esc_attr( sprintf( __( 'Permalink to %s', 'twentyten' ), the_title_attribute( 'echo=0' ) ) ); ?>" rel="bookmark"><?php the_title(); ?></a></h2>
		<?php the_excerpt(); ?>
	</div><!-- .col-md-9-->
</div><!-- .entry-content --><hr>
</article><!-- #post-<?php the_ID(); ?> -->

Here is the screenshot of my archive page