Installation/ATLauncher

Steps
Download the ATLauncher for your platform at this link. If you're on Windows, pick the Setup option.
Find the downloaded file, and run it. The folder button will show where the file is.
If warned by Windows, click "More info" and then "Run anyway".
If using the Windows Setup method, proceed with the installation. Follow this link if you need specific instructions.
Select your language, and then hit "Save" when the setup window appears after launching ATLauncher.
Click "Accounts" and enter your Minecraft account details, then click "Add". If you have a Microsoft account, log in via the matching button instead.
Click "Packs", find the Reforged pack and click "New Instance".
Select your preferred version, then click "Install".
Click "Instances" and click "Play" on the Reforged instance!