Now that you've gone through the steps on the 'Creating a Playlist' article, you can keep adding songs to your playlist by doing the following:
If you listen to a song you like, click on the three dots to the right of the song name
This will open this tab:
By clicking on add to playlist, you'll be shown a tab with your playlists
Choose the playlist you want to add the song to, click on it
And that's it! Happy listening :)