MultiMC method

This one is much simpler and less convoluted, which is why i recommend you use MultiMC if you love modded Minecraft. Select the Minecraft version compatible with the Aether version you want to download. Select your instance then go to "Edit Instance". once you did that, click "Install Forge" and install the latest version. Now all you need to do is go to "Loader mods" and click on "Add" then select your jar.