For information on what Ritual Magic is in this campaign, see the Path and Book Magic page.
No matter which method your character uses to learn mages, here is how you work out character points associated with Ritual Magic:
- Take at least  Ritual Magery 0. This represents enough resonance with the Spirit World to access Paths or Books. Magery higher than 0 represents greater resonance with the spirit world, and thus the ability to do more powerful things.
- Pick a Method for learning your Ritual Magic,and buy the appropriate prerequisites
- The Traditions: Tradition Investiture 
- Deserter: Unusual Background (Deserter)  + [-15] in Social Disadvantages
- Rogue Element:  Illuminated advantage
- Grimoirist: Perk (Grimoire Exposure)  + a Native-level Magical Language  +  in academic skills
- Reawakened: Reawakened 
- Betrayer or Chosen: Supernatural Patron  with +100%: Teaches Supernatural
- Take at least  in Ritual Magery (IQ/VH).
Now you are a Ritual Mage. You can now perform any Ritual based on (Ritual Magery -6 -(Ritual Default from Path)).
Example: The Chaperone Ritual (GURPS Thaumaturgy p154) defaults at Path of Luck -3, and Path of Luck defaults to Ritual Magic at -6. So a Ritual Mage with Ritual Magic 16 could perform Chaperone at a 7!
Most often, defaults will leave you with a very hard roll to make!
So, to improve your Efficacy with Ritual Magic, take Path skills (IQ/VH) for any of the Paths listed on Path and Book Magic. You can improve this based on the -6 default from Ritual Magery, but it may never go above your Ritual Magery skill.
Then, take Hard Techniques for each Ritual you would like to perform.
Grimoirists use Book Magic instead. The only difference here is that there is no default to Ritual Magic (though the Book skill cannot exceed Ritual Magic). A Grimoirist must learn the Book skill (IQ/VH). Each ritual is at a default from that Book skill.
Leveled Costs and Multipliers Costs
There may be some additional costs associated with your Ritual Magic skills, based on the Methods of Magic. Calculate those costs as necessary.