There are a few ways to link your previous post. First you can go to your old post and copy the URL from the brower. Then go to the text box for the current day, and you can select the link icon (7th from the left) in the tool bar above. It'll give you a pop up where you can enter the URL and that will give the link like I did above.
The slightly fancier way is like what you did to create the link to Kole's spreadsheet in your signature. You copy the URL like you did above, then type something like "yesterday's link", then highlight those words and select that link icon again. That will do the pop up again, but when you enter the URL, will make those words you highlighted become the link, instead of the long URL like I did above.