There are 4 primary methods to gain access to a spirit formula: random discovery, a hunt for the physical anchor, a metaplanar quest, and an effort to design the formula.
Random Discovery: By the providence of the GM, a player can gain the spirit formula or true name of a spirit. This is usually done in connection with a plot or as a reward for a job.
Hunt Down The Physical Location: A spirit's True Name becomes physically imprinted on an object, a person, or even a place that was near the spirit when it first broke free. The physical true name is similar to an astral signature and can be identified with an Astral + Intuition (3) test once located.
Translating the True Name into a spirit formula requires a little time and effort with an Astral + Logic (Spirit's Force x 5) extended test that takes 1 full day.
Metaplanar Quest: An initiate or individual with access to the metaplanes may learn a spirit's true name by undertaking a metaplanar quest to the spirit's native metaplane. The spirit's metaplane can be obvious or obscure, and without obvious clues the magician must assense the spirit or its signature. This requires either an Astral + Intuition (4) test if observing the spirit directly or an Astral + Intuition (5) test if assensing an astral signature.
This is a harrowing, difficult undertaking and an adventure of its own. The caster likely will require a guide and at the end of a success quest, the true name is etched into the initiate's mind. They can then translate the True Name into a spirit forumla with an Astral + Logic (Spirit Force x 3) extended test that takes 24 hours.
Design Formula: If a magician spends time directly observing a spirit, they can perform an Astral +Intuition (Spirit's Force x 5) extended test that takes 1 hour. Success allows them to attempt to design a functional spirit formula from their impressions of the spirit.
Creating the formula requires an Astral + Logic (Spirit's Force x 10) extended test that takes 1 day.
If the test is interrupted in any way, it needs to be completely restarted. A Glitch means the magician thinks they have made good progress but instead they have introduced an error that will make the formula a dud. A Critical Glitch means they have someone's name, but not the intended spirit... and when they show up, they'll be uncontrolled and very pissed off.