How do I add notes to hours in the timesheet?

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

A yellow box will appear where you can edit the note.
Hit Tab or Esc to close the box.

You can also add a note with the mouse.
Locate the yellow icon that appears in the top right when hovering over an hours field, and click it.

In the Daily and Monthly Timesheet views, the note field appears by default.