Open up the menu, click on Preferences > Recommended Software, and then select Scratch 3 and click on OK.
Open a terminal window, and type in and run the following commands:
sudo apt-get update sudo apt-get install scratch3
Once Scratch 3 is installed on the Pi, go ahead and open up Scratch 3 by clicking the Raspberry Pi icon on the top left of the desktop, then Programming>Scratch3
Click the blue icon at the bottom left of the application to open the extensions.
Select the extension entitled "Raspberry Pi GPIO"
Now we have access to some awesome GPIO functionality which we'll get to next!
Read more about the different Raspberry Pi extensions for Scratch here!