Becoming

This information can be found in Geist: the Sin-Eaters
Quick info about applying the Geist Template, along with some commentary:

Notes on Vocabulary

Geist – synonymous with Sin-Eater. Geist (capitalized) is the combined and symbiotic entity, while a geist (lower-case) is the hitch-hiking entity which struck the Bargain with the Bound.
geist – the entity tagging along in the Sin-Eater's body. Not a ghost, not quite a spirit. Something different altogether.

Choose your Archetype

Choose your  Threshold

Choose two Keys

  • One must come from your Threshold
  • The other might be influenced by your geist

Choose your Manifestations

Record your Psyche

Re-allocate Merits, if desired and appropriate – remember to look at Ceremonies
Design your Keystone (see below)

 

Other possible changes:

You are welcome to use the event of your death as a reason to resolve any conditions or tilts you may be suffering.
Of course, it is also completely possible that dying has not made your character any more sane or stable (lookin' at you, Max).

Keystones

"As part of the bonding process between geist and human, the Sin-Eater is given a gift to symbolize the bond. This gift is a keystone memento — nothing less than the unbeating heart of the geist. It is a physical manifestation of the geist’s past and the key to its power.
The act of offering this precious object to a Sin-Eater is a declaration of empowerment, the handshake of a partnership and, in part, a demonstration of submission. Without a body to walk and talk, and provide a physical anchor for them, a geist is little more than an exceptionally powerful ghost. By offering up its keystone, the geist acknowledges the value of the human with whom it has bonded.
Each keystone is different and the form it takes bears direct significance to the history of the geist, even if the geist can’t remember exactly why that might be. The waterlogged specter of a geist, dripping with seaweed, its face a craggy mass of scars, might not remember why its keystone is a polished, ivory tusk, but that in no way diminishes the power of the memento. More often than not, though, the connection between a geist and its keystone at least seems obvious. The keystone of the Burning Woman — a geist that appears only as a mass of twisting flame and the scent of burnt flesh — is an ever-burning torch that can’t be extinguished by any known means. The keystone of a geist with the sunken features of a starvation victim is a wooden plate, upon which any food placed instantly rots and decays.
Keystones have a Threshold and two Keys that relate to the geist of which they are a part. These signifiers present an opportunity to more fully flesh out how the keystone relates to the geist and to the Sin-Eater to which it now belongs. The ever-burning torch of the Burning Woman has the Threshold of Death by Violence along with the Elemental and Passion Keys. What this indicates about the Burning Woman is that she was the victim (or perhaps the perpetrator) of violence (the Torn), relating to the use of fire (Pyre-Flame) and was either possessed of, or the target of, strong emotions (Passion).
Likely, the Sin-Eater that has bonded with the Burning Woman shares one or more of these signifiers, even if she didn’t before. The Threshold and Keys needn’t exactly match those of the geist or the Sin-Eater, but like attracts like. Sin-Eaters that enact Manifestations that share a Key with the keystone gain a +3 bonus to the roll.
Ephemeral Objects
Although technically given over to their Sin-Eater during the bonding process, keystones are generally kept safely in Twilight as part of the geist. Sin-Eaters can retrieve a keystone from Twilight with a thought (reflexive action) and returning a keystone to the geist is nearly as easy (instant action). A Sin-Eater isn’t under any compulsion to return a keystone to her geist and may keep it as long as she likes.
While in the flesh, so to speak, keystone mementos are easily recognizable by other Sin-Eaters, geists, and even ghosts for what they are. The almost palpable aura of death that surrounds a keystone is unmistakable as anything else. In this way, keystones act as something of an identifier among Sin-Eaters. Anyone could potentially own a deathmask (see below), but only Sin-Eaters become resonate with an active keystone, which signifies the presence of a geist.
As has been stated, keystones generally exist in a state of Twilight, part and parcel of the geist of which it is a part. When a Sin-Eater retrieves a keystone, he forces it to manifest in a physical form. The transformation from intangible to physical is instantaneous, but does come with some drawbacks. Most keystone mementos are small objects (Size 1 or 2) that have no difficulty making the transition. Larger keystones lack the ability to become completely solid for anyone except the Sin-Eater to which it belongs.
Keystones of Size 3 or larger (maximum of Size 12) are completely ephemeral to anyone other than their Sin-Eater owner. The keystone can be seen, heard, and even smelled by anyone that could normally interact with it, but remains as solid as smoke. Only the owning Sin-Eater can interact with the keystone as though it were a physical object.
This obstacle can be temporarily overcome by reinforcing the structure of the keystone with plasm. A Sin-Eater can force a complete manifestation of a large keystone for one scene as an instant action by spending a point of plasm. Keystones that are made to manifest in this way may not exist in the same place as another physical object until they are fully formed (i.e. a Sin-Eater couldn’t solidify the keystone through another person or object). Manifested keystones come to rest in a natural position that obeys the laws of physics, so while it is possible to manifest a large keystone in such a way so that it will become unbalanced and fall on an enemy, Storytellers should require an opposed Intelligence + Science versus Wits + Composure roll to do so. Fully solid keystones have the same Structure and Durability as other, similar objects and if they are destroyed will return to Twilight. Keystones that are violently returned to Twilight may not be retrieved again until the end of the current scene.
It should be noted that keystones are bound to a Sin-Eater and her geist by more than just physical proximity. It is impossible to steal a keystone or for a Sin-Eater to misplace or lose a keystone as long as she retains her bond with her geist. Manifested keystones can be destroyed (as described above), but that only denies the use of the keystone to the Sin-Eater for a single scene. Keystones that are somehow misplaced by their owner can be recalled to hand as a reflexive action, similar to the manner in which a Sin-Eater retrieves a keystone from Twilight. Only the keystone mementos of geists that have been defeated and destroyed (see deathmasks) can be stolen, traded, or lost.

Storyteller's Interpretation:

A keystone is an object that should be tied to the history of your geist. It is not something that should have belonged to your character, unless somehow your character and your geist have been in contact for the duration of your character's life.

Becoming

And Justice for All yskah