To begin the content of Ars Plus you'll want to hunt the Mysterious Mages that have appeared across your world and obtain their Mage Cloth. Once you're ready, craft a ritual of Mage Summoning to challenge the strongest of mages within their structures and obtain their magecraft.
If you'd like to submit a custom Mysterious Mage of your own, message @th3S3nate on the Ars Nouveau discord under the Ars Plus channel :)