Step 1: - Download the required files retrowrapper.zip and resources.zip.
Step 2: - In the official and most recently updated Minecraft launcher, be sure to have "Historical Versions" checked under the "Launch Options" tab. Create a new profile and under the "Version" dropdown, select "old_alpha a1.2.6". Go back to the "News" tab, launch the game, and close it after it loads up fully.
Step 3: - Extract and run the "retrowrapper-installer.jar". Make sure "old_alpha a1.2.6" is selected and then press "install".
Step 4: - Extract the "resources.zip" folder into the root directory of your ".minecraft" folder. If you are unsure how to reach this, try holding the "Windows key" and then press "R". Type in "%appdata%/.minecraft" and then press okay. This should bring you directly inside the folder.
Step 5: - Start up your Minecraft launcher. Edit the existing profile you created, and under the "Version" dropdown, select "old_alpha a1.2.6-wrapped". Go back to the "News" tab, launch the game, and you're ready to start playing!