We'll assume transformation via "The Ghoul's Manuscript", so let's start with the book itself. There are two versions to choose from:

The Ghoul's Manuscript
by Scott David Aniolowski
Source: TUO#4 pg43-44, or see its online description here

This version of the book is statted as follows:
+5 Mythos, -1d10 SAN; Spells (INTx1): Call Mordiggian, Contact Ghoul, +1d3 others
-1d10 SAN when finally succumbing to necrocannibal urges after # days
1d6 days to transform, -1d10 SAN/day until transformation is complete

Average SAN loss using this method is between 20 and 45 points.

See also Dark Denizens of Dreams and Beyond: The Mythos Additions of Clark Ashton Smith, also by Scott David Aniolowski. Especially "The Great Ghoul Mordiggian, the Charnel God (Great Old One)".

A second version of "The Ghoul's Manuscript" appears in the DELTA GREEN sourcebook (PAG1005), p058:
"The Ghoul Manuscript"
Lang: English (sub literate)
Mythos: +5
SAN: -1d4/-1d10
Spell Multiplier: x1
Study time: 52hrs
Spells: Call Mordiggian, Cloud Memory, Consume Likeness, Contact Ghoul, Enthrall Victim

No notes on SAN losses due to a growing hunger for corpses and the resulting physical transformation, so use the online version if you wanna go that route.


Nancy
What sort of notes on the "Ghoul Manuscript", ghouls and the Dreamlands has Agent Nancy put together for A-Cell? See pg57 for info on Agent Nancy and her ghoulish NPC stats. Pg13 gives us slightly dated ghoul "big picture" info in the mid-1990s.

There's an account of Debra Constance/Jean Qualls/Agent Nancy's transformation in the novella "Identity Crisis", which is in the sadly out-of-print "Alien Intelligence" collection.

Here's what I worked out for the actual transformation:
ghoul stat changes:
STR: +6
CON: +1d6
POW: +1d6
DEX: +1d6
SIZ: +1d6-3, min roll = 0 for no change
INT: unchanged
EDU: unchanged
APP: -2d6

ghoul attacks:
-  both claws & bite in a single round
-  if bite strikes home, can hang on instead of using claw attack & worry victim with fangs for 1d4 damage automatically; STR vs STR to dislodge what amounts to a successful Grapple & end Bite damage

ghoul weapons:
   Claws 30%, damage 1d6+db
   Bite 30%, 1d6 + automatic worry

ghoul armor:
   firearms & projectiles = 1/2 rolled damage, round up fractions

ghoul skills:      [human base]
   Burrow 75%
   Climb  85%      [40%]
   Dodge  DEX*2 (start)
   Hide   60%      [10%]
   Jump   75%      [25%]
   Listen 70%      [25%]
   Scent decay 65%
   Sneak  80%      [10%]
   Spot Hidden 50% [25%]

Dreamlands skills:
   Dreaming: starts = to POW
   Dream Lore: starts at Mythos/2

I came up with this by comparing lesser ghoul stats with newly rolled PCs using the v5.6 BRP rules, and then consulting the Pagan materials to get a feel for fleshing out the numbers, especially Agent Nancy's scenes in "The Rules of Engagement" novel.


Sources
Some other Pagan Publishing ghoul and Dreamlands mentions (because ghouls & the Dreamlands are so intertwined) include:

DEVIL'S CHILDREN (PAG1001)(oop)
p038-039: Arkham/Miskatonic U. ghoul tunnels mentioned

THE GOLDEN DAWN (PAG1004)(oop)
p058 - Oak Wood in Northern England; St. Michael's of the Wood
"...a place of unbound magic, much larger within than it appeared from without, impossible to map and inhabited by unearthly spirits."
"A great Elder Sign was ensorcelled around it."

A place where the Dreamlands' Enchanted Woods touches the waking world? Along with Kingsport? And Leng? Plus various ghoul warrens?

REALM OF SHADOWS (PAG1006)(oop)
A whole damn campaign book dealing with ghouls and ghoul cults! Yippie!
p008-009: New spells
p010-011: Ghoul info
p018-024: Cult of the Charnel God
p025    : Cultes des Goules
p112-113: Dreamlands
p124-139: Zothique (note the larger red sun; SAN loss 0/1?)
p170-173: Dreamlands temple and gates
p173    : Ceremony/Talking Skull
p184-187: rites

MORTAL COILS (PAG1007) Buy
pg46-48: Ghoul info in "A Murder of Crows"
pg71-91: Dreamlands-related scenario "Nightcap"

Do any of the artifacts mentioned in "Nightcap" still exist? A partially translated tome, giant rubies,  some Moonwine... here did the Kentucky cult get the Gate of Oneirology spell? What happened to the "spell enabling Dreamlands items to be brought back to and exist permanently in waking world after appropriately hideous rituals"?

DELTA GREEN: COUNTDOWN (PAG1008) Buy
p058-087: GRU SV-8; they don't like ghouls.
p160    : Consume Memories spell (Tiger Transit/Tcho-Tcho; possibly a humanized ghoul ritual?)
Temporarily absorb the memories & skills of whomever's fresh brains the caster consumes; expend 10MP while devouring the still-warm brains of a freshly dead corpse; caster may use any and all skills and knowledge the victim possessed for the next 24hrs; 10POW to permanently absorb (inc spells). 1d10 SAN for either version.

I'm still working on a Nemesis conversion of the BRP ghoul conversion above, but this is what I have so far:

Ghoul transformation (Nemesis)  
+2d: Body  
+1d: Coordination
+1d/2d: Sense
unchan: Mind
-1d/2d: Command
+0d/1d: Empathy

+1d to Equilibrium *after* MM hits?

MM hits
Unna: 6-7? become ghoul, consume memories & appearances (5=see Deep One, 6=see Serp, 7=see ET)
Help: 7-9? become ghoul (7=waiting on death row, 8=herded to death, 9=confirming Mythos)
Self: 5-6? hiding truth from gen pub (3=steal from family, 5=defraud everyone you know)
Viol: 5? eating corpses (4=see mutilated corpse, 6=attacked by undead)

+1d/2d to mythos


The process of becoming a Ghoul
By Agent Donald

Say a PC has read Cultes des Goules or the Ghoul Manuscript, what happens next?

I guess it starts with the PC having nightmares where he is eating human meat. After a while he will get this craving in the real world. The PC will start seeing other humans as food.

After his first consumption of human meat, his skin will slowly turn to a greyish, white color like an old corpse. The PC will notice that he becomes stronger and that he can see better in the dark. This will give him a rush of superiority and he will want to become "better". As he eats more and more 'white pig' he will become faster and stronger, he will start to smell like a dead body and he will be able to see in pitch black. He will know that he is vastly superior to human beings and he will only see them as food.

All this time he is having nightmares that teaches him more and more about the ghouls, their habitat and how they live. The PC will notice that in the evenings homeless people follow him or crowd around his place of residence.

In the mean time, friends and relatives will have left him, he will be unemployed and more and more is he attracted to solitary spots like cemetaries and sewer tunnels.

Than one evening, he will hear a knock at the door. On his doorstep stands a ghoul dressed in rags who tells the PC that he must follow him as "They" are coming to arrest him. The neighbours have complained about the smell coming from his house. The PC knew that it was a matter of time until the bodies in his house and cellar would be noticed.

If he decides to follow the ghoul he will be accepted in their ranks. If he refuses than in a couple of minutes police cars arrive and the cannibal-PC will be frontpage news for a couple of weeks.