When you are in the campaign designer, simply go to the top left of your campaign, and hit the arrow down button, and push notes.
Now you will see a new screen looking like this:
At the bottom of the view, you can write a note for yourself and hit the post button. After hitting post your message will appear like this.
Your can add as many notes as you like and mark them as done when you have completed the task. If you do not need your note anyway then simply press on delete.