Step 1: Open your Minecraft Launcher.
Step 2: Click on "Launch Options" and then click "Add new".
Step 3: Name your profile whatever you'd like and choose the version you want to use (1.12.2).
Click Save and return to the "News" tab.
Step 4: Click the arrow next to "Play" on the bottom and select the new profile, then press "Play"
Step 5: Enjoy DarCraft!
You can switch back to your previous profile by simply pressing the arrow again and selecting "Latest Release".