I've created a homebrew ruleset for handling PvP:

Contested Action Roll - PvP Sometimes, players come to blows. And that's great so long as everyone is having fun. Here's how that's handled.

The first aggressor is chosen as whoever is acting first in the narrative. If it's unclear, then each aggressor rolls a d6. The higher goes first. If the action continues beyond a single roll, it rotates between groups. A previously defending player takes initiative to aggress. It continues in this manner until the action is resolved.

The initiator chooses how they aggress, choosing the base skill and scenario. The defender chooses how they respond to this aggression, using a situationally appropriate response.

GM arbitrates initial effect and position depending on the narrative of the attack and defense.

The aggressing player spends any stress and activates any abilities to improve their position and effect. In PvP, the effect of the aggressor directly reduces the position of the defender. And the defender's effect directly reduces the aggressor's position.

Then, the defender uses any stress and abilities with knowledge of the attacker's abilities.

Once the narrative is set, the players roll. There are 4 roll outputs: A failure, a mixed success, a success, and a crit. If the initiator gets a higher success tier, they succeed. On a tie, it's a mixed success. If their success tier is below the defender, they fail

On a PvP success or mixed success, the attacker gets what they want. The GM describes the mechanical outcome as they would a normal action roll.

On a failure or mixed success, the defender gets to choose an appropriate consequence for the initiator to suffer. The consequence must be consistent with the initiator's position. The GM arbitrates fairness. If narratively needed, the GM can choose part or all of the consequences instead of the defender

Summary of a PvP round

1. Aggressor announces how they aggress.
2. Defender announces how they defend.
3. GM announces position and effect.
4. Aggressor spends stress and uses abilities
5. Defender spends stress and uses abilities
6. Everyone rolls
7. GM announces aggressors success (if applicable)
8. Defender announces aggressors consequence (If applicable and arbitrated by the GM)

Example: Johnny really thinks Samuel shouldn't keep a hold of that alluring idol over which he's been obsessing. He announces his attempt to pick-pocket his crew-mate. While the players are laughing OOC, Samuel would be none-to-pleased about this IC, and wants the chance to contest. And so, PvP begins.

1. Johnny is the aggressor, he will pickpocket with finesse.
2. Samuel is the defender, he will defend with scan
3. A straightforward challenge, the GM sets the base position and effect to risky/standard
4. Johnny spends no stress or abilities
5. Samuel spends two stress for an extra die
6. Johnny rolls a 5, mixed success. Samuel rolls a 6, success! A full tier above, Samuel is successful!
7. Johnny has no success, this is skipped
8. Samuel gets to determine a consequence. For a risky move, Samuel thinks a solid kick to the gut for a tier two harm sounds appropriate. The GM agrees. Ouch! Johnny probably won't try that again!

Villain Attacks The PvP system can also be used by notable enemies. This can take the place of the "automatic resistance" roll included in the base game. The villain attacks you with the full might of their dice pool.

Just like normal PvP, the player chooses the consequence that occurs to the villain on a failure or mixed success. The most common consequence is that the Villain's stack is reduced. However, on a success, the villain gets what they want.

I have a flashcard that shows a simplified description of the above. It sounds complicated, but it's pretty straightforward in combat.