To show a specific selection of your posts in the sidebar, contrary to what some top-ranking search results will tell you, you don’t need to install yet another spammy WP plugin or write any HTML.

All you need is the standard Text widget.

Here’s how to add your own “selected posts” section.

Steps:

1. Go to Dashboard > Appearance > Customize
2. Click Widgets > Main Sidebar (this part may vary depending on your theme. I’m using currently Astra)
3. Click Add a Widget then “Text – Arbitrary text”:
   
4. Enter “Selected Posts” for the widget title, or something similar
5. To add posts, click Insert/edit link then start typing the title of one of your posts. WordPress will auto-suggest matching posts in a drop-down list.
   
   
   Note: you can customize the link titles in case the default ones include odd formatting characters.
6. When you’re done hit Publish.

The style of the final result should match your Recent Posts widget (if you have one) exactly:

I hope this is of some use. WordPress has a thriving ecosystem of plugins but many of them are spammy at best, predatory at worst, and (in this case) completely unnecessary.