Adding icon/button to sidebar top icons (like RSS)?

I would like to add an icon/button to sidebar top icons, similar to how there is a RSS icon and a bookmark icon etc.

It was probably already answered a few times, but I can't find an easy explaination.

Thanks for the help.

Beginning Developers

Beginning Developers

This space is for newcomers, who wish to build a new plugin or to customize an existing one to their liking