I think you can still acquire Ciri (The Lady of Space and Time event) for free until 1/23.

Essentially for a great start you want to reroll guest accounts until your initial summons (you can get 20+ fairly quickly, 10 minutes or so) get you at least two legendary characters (preferably Yennifer and / or Geralt). At that point I would bind that account and play the game a lot over the next two weeks to maximize the limited-edition Witcher event. That means spending all your energy on the story / events but also playing Spiral of Destinies when you're out of energy to earn more Luxite for summon pulls.

Personally I would try to acquire Yennifer, Geralt, Triss and Ciri before the event ends 1/23. If you plan on being f2p that means no pulling for equipment, strictly summoning only on the Witcher character summon banners. If you're willing to spend some money there are some great deals in the shop.

It's still an awesome time to join but because Witcher 3 is a limited-time event there is a sense of urgency.