Edit default Sidebar of Files in Ubuntu 19.04

I can't edit the sidebar of Files in Ubuntu 19.04. I saw many answers but they were quite difficult to understand, and the easy ones didn't work at all. I have other folders which I use frequently, so I want to have them in the sidebar.

There are different sections in the left sidebar of files, separated by a faint gray line. The top section contains your special user folders. This section is not intended to be edited, but you still can by editing a config file, as explained in How to remove bookmarks from the Nautilus sidebar?.

Under this, mounted network shares appear.

The third section is where your bookmark section appears. Here, you can add your favorite folders as explained in How do I add folder to Ubuntu 18.04.1 Nautilus Bookmark list?. Folder you add there can be easily removed again if you wish by right-clicking the item and selecting "Remove".