The simplest way to do this is create a new sidebar and make the following changes to the archives.php file.
First, do a Save As of the sidebar.php file. Save it as sidebar-for-archives.php. Now you have a file to include.
Next make the following changes.
1. Line 9 of archives.php: Change <div id="content" class="widecolumn">
to <div id="content" class="narrowcolumn">
.
2. archives.php: Right above the last line (<?php get_footer(); ?>
), insert this as a new line and add <?php get_footer(); ?>
.
These changes will do two things:
First, you will tell archives.php to use a narrow column, which will allow room for a sidebar.
Second, you will tell archives.php to include sidebar-for-archives.php as its sidebar.
You can then customize sidebar-for-archives.php any way that you like. I have removed the code in this file that shows archives by month and category, because archives.php is already doing this.
You can do this on any page, really. I am even doing it on my individual post pages to include a different sidebar than the normal one on the index.php page. I wanted one that was shorter, because I include long advertizing on the index.php sidebar, and this looks bad on a page with a short post.
Good luck! Read the WordPress codex (https://codex.www.ads-software.com). There is a section on customizing the sidebar (just type “customizing the sidebar” in the search field). That’s where I learned how to do this.