While in the detail screen at some item, e.g. wages, click on the Note icon across the top and drag it down to that input area. Type your note and click OK. When you are done you will see a little note thingie at that input and you can click it to see the note, or go back to the upper icon and click it. 

@IntuitBettyJo can maybe fix the broken link at the bottom of https://proconnect.intuit.com/community/print-a-file/help/creating-client-notes/00/4277 for sticky notes.