Mr. Monte has some words on this very subject here:
https://www.montecookgames.com/monte-says-pcs-versus-npcs/
Mainly talking about game design, and designing creatures from the ground up, and touching on his previous game experiences. Sadly if you are like myself you might find it a little of a let down, as he explains that converting anything into NPCs or Cypher NPCs as more of an art than a science. That said, I do have some experience converting PCs into NPCs as a very very simplified conversion method that met with passable success.
Creature Level
Starting with the base that “3” as the standard difficulty most Tasks start, as defined under the Task Difficulty Scale. The only other defined level of power a PC has is their Tier level.
A PC being converted gains a level of 2 + the PCs Tier.
Making Tier-1’s level 3, and Tier-6’s level 8.
As per normal for creatures their HP is often the standard x3 their base level if they are considered “average”, x4 if they are extra hardy or experienced combatants, and x5 if they are truly a threat, and maybe a little extra if they had a really high Might Pool.
(x6 Hp is often reserved for level 6+ threats like Omega Destroyers and Dragons).
Skill Abilities Modification
For skill and abilities adjustment into modification you simply take whatever a character is trained (+1), specialized (+2), hindered (-1) and apply those adjustments against their level as you do normally with Creature modification.
Tools and equipment that were central to the character were included. However all others minor equipment and all Task Familiarity bonuses are excluded/removed.
Type, Pools, Armor
Type can largely be ignored as that's more of a tool used by players to focus on what pools or abilities they want their character to be good at. If you feel the previous PC had particularly high Might or Speed feel free to get them +1 armor, and one additional +1 armor for each Edge in Might or Speed. (Edges in Int I had as a modification against mental/spell/Int based attacks)
Descriptor and Focus
The Descriptor and Focus become alot more loose in what to do with them. I would take which ever feature felt was the “main reactive ability” or the character's most debilitating ability and used that as the default Intrusion when confronting that NPC.
Creature Combat
For converting PC Combative Abilities to Creature Combat I summed up a quick list of notable abilities granted by the Type, Descriptor, Focus. Anything which actually granted a benefit to attack became +1 difficulty for the PCs to defend against. Any extra damage became simply a +1 to damage to the appropriate pool, and non-straight forward stuff you just have to wing it. Abilities that overlapped with others were discarded to help pair down the list.
Many advance level creatures in Numenera/Cypher often sport the "This ability cannot be used in consecutive rounds." to balance powerful effects. Apply this to any ability that is normally tier 4 or higher. In practice I've never used any combination of tier 4 abilities consecutively.
In addition one of the better resources I found useful for modifying monsters is CypherSystem's Predation setting which has a "Common Creature Upgrades" table which details several one off buffs for monsters. I'm not sure what other Cypher Systems use a similar adjustment table, but that one as served me well when I want a creature to feel more unique.