Create a good looking side bar with thumbnails

See if this works for you
https://www.ads-software.com/plugins/recent-posts-widget-with-thumbnails/

Thank you, but as I am writing code I don’t want to use a plugin for this. Is there any way out?

Oh! Most people just want a plugin. Good for you.

Learn how to do that here.
https://codex.www.ads-software.com/Post_Thumbnails

If you don’t mind please have a look here: [ redundant link removed ]
This is what I want to create. But in the functions.php I see before title and after title keywords. I have already gone through the page you have shared.

I don’t need to see an example. I know exactly what you want, but now I feel you want someone to write the snippet for you? Is that correct?

No! I was asking is there any keyword to assign in the functions.php so that I can write my html code inside that.

Uh, keyword? I did not see a request for keyword and at this point I’m not even sure what you want. You’re all over the place.

Moved to Fixing WordPress, this is not a Developing with WordPress topic.