You asked for the joining technique used on the Lucky Thirteen Granny blanket and I’ve got the answer! It’s called a PLT join, in this case I did it into the back loops only, and rather than working around each square, I did long horizontal and vertical rows to join (I find this method much faster).
If you’ve ordered the pattern, you will get an update with this link added so you can download the tutorial separately since it’s so many pages.
If you’d like to follow along with the tutorial here, great! But if you’d like to download the PDF version of the tutorial, please use this link: PLT Back Loop Join