How to add notes to hours in the timesheet?

In the Weekly Timesheet, type the hours for a day.
Then start to type the note in the same field, or hit the space bar.

A yellow box will pop up to edit the note.
Hit the keys Tab or Esc to close it.

Another way is to use the mouse.
Locate a yellow icon appearing on the top right when you go over an hours field.
And click it.

In the Daily and Monthly views of the Timesheet, the note field is appearing by default.