You download the .rar file and then extract it using 7-Zip or WinRAR into the Blockland folder.
.rar = Compressed archive.
.bat = MS-DOS like code which runs on click.
The .bat you may have heard being talked about is how you launch Blockland with the editor, not how you install it:
Start a new Notepad document.
Type this code:
start blockland.exe -mod editor
File --> Save As...
Save As Type: All Files
Save it in your Blockland folder as Editor.bat
Click this file instead of Blockland.exe
