Drag the "say in chat" block and attach it under the "connect" block.Go to Minecraft in the left bar, and drag "connect to Minecraft" to snap it under the "when green flag clicked" block.Go to Events in the left bar in Scratch, and drag the "when green flag clicked" to your programming area (the area with a dotted grid).Go to a browser and click on this link to go to a version of Scratch 3.0 with the RaspberryJamMod extension preloaded. Run Minecraft and a browser in separate windows.
Start up Minecraft and create a superflat creative world (easiest for programming in) with Forge and the RaspberryJamMod mod active. This should run a Python script that creates a big glass donut filled with water. Start a Minecraft installation with Forge for 1.12.2, start a new single player world (creative and superflat are recommended), and type /py donut and press enter. If you've got the Python support, you can do a quick test that the mod is working without interfacing with Scratch.
If you want the Python support, download the Python scripts package and put the contents (an mcpipy subdirectory) in your. minecraft directory ( %APPDATA%\.minecraft on Windows), create a mods subdirectory, and copy the contents of mods.zip into there. If you are on a different operating system (or like to do things manually), then download mods.zip instead.
This will also install Python in addition to the mod, allowing for more advanced Minecraft programming once Scratch is outgrown. On Windows, you can just download the easy.
You can get RaspberryJamMod from its releases page. Note: Microsoft has their own block-based programming system for their Windows 10 Minecraft: Education Edition. This Instructable covers installation and setup, and describes a number of example programs. If you want to do that, there are video instructions here, and you can skip the two installation steps in this Instructable. You should also be able to use a Spigot server with a RaspberryJuice plugin instead of Minecraft+Forge+RaspberryJamMod.
However, I will include an appendix explaining how to use the Minecraft Scratch extension with Raspberry PI Minecraft, together with a Python proxy script. RaspberryJamMod only works with Java Minecraft 1.12.2 (and some earlier versions). I assume you already have Minecraft installed. These instructions will be mostly designed around Windows but should work on any other system that can run the Java version of Minecraft. I now wrote a browser-based Scratch 3.0 extension that interacts with RaspberryJamMod and lets you (or your kids or students) make block-based programs that run in Minecraft, including turtle graphics. Some years back I wrote RaspberryJamMod, a mod designed for programming Minecraft in Python.