Answer: A lot of work.
Here is an example
When I was first building this site, I threw up a Recent Posts widget in the right sidebar. It has bugged me ever since.
As noted in the screenshot, the article topic is a social media topic, but the first article linked in the Recent Posts is a programming topic.
What if I could show posts from the same topic in the right sidebar?
I can! … with a plugin.
My website has a feature that is not about the viewer; it’s about the technology. On each article page, there is a Recent Posts section in the right sidebar. But, like a department store, my site has a lot of topics that are relevant to different viewers.
First: How Did I Get Recent Posts into the Sidebar?
Recent Posts is a widget that is available in WordPress by default. That means that it’s a core function.
Most WordPress themes have sidebars registered as part of their default. Kahuna is the same, in that it has a right sidebar available. The WordPress core has functionality for themes to add (register) as many sidebars as the author wants. You can see a list of the sidebars and widgets available by going to Appearance -> Widgets in the Dashboard.
Second: Finding a Plugin
The next step was finding a secure plugin to do the job. I did a Google search and the top two results were for plugins that are out-of-date. You can see from my search, that I should have used the word Category instead of the word Topic. The problem here is that search results are based on what they know and put on their websites, not on what you know or how you think about the topic. Google has no skin in whether plugins are up-to-date or not.
So, I switched my tactics and did a search on WordPress.org, instead. WordPress does have an interest in featuring the best plugins. The WordPress search shows how up-to-date the plugin is right in the search results.
After looking at the descriptions, I chose to install Category Posts Widgets.
Third: How to Use the Plugin
Before I add another widget to the sidebar, I need to remove the current 
Recent Posts widget to avoid confusion in not having two “Recent Posts”. That also means that I will have to go back through all the Posts on the site, one-by-one, and add a widget for the specific category. Until I do that, there will be posts that don’t show links to any posts in the right sidebar.
But, it took me two attempts to make this plugin work. It’s a UX problem in the dashboard. I could make the widget show on the pages I wanted, but I couldn’t get it to only show posts in the Category I wanted. Why? It’s a matter of the most important function being hidden amongst a bunch of functions I didn’t care about so much.
One of the first things I noticed in the interface is that the Hide/Show function is the same as it was for the default Recent Posts from the WordPress core. That’s good! Always build on what users are already using, unless there is a compelling reason not to.
But, what was my main goal in choosing this plugin? It was to choose a set of posts to link to, based on the Category. And, they give me Title and Filter and Post details and General and Footer … and all the stuff further down. So, in the process of investigating all this stuff, I missed what I was really looking for: Filter. I was looking for Category. They called it Filter. Both are true, because the function filters the category!
But, in my hurry, I saw the word “Categories” below so I tried to filter there. When I went to check the posts were from all categories, but the list only showed on the Category pages I had chosen. That is because there are two functions where you choose a Category, 1) the list you want, 2) the places you want that list to show.
Other Stuff
This plugin has a lot of other features that are more involved than what I am likely to use, at least in the near future. If I were choosing this plugin for a client, I would be careful to consider whether the client or their staff members would be confused in all the options.