i believe the garrote was in the old Songs and Silence book (3 ED)
Basically,
Step 1
You provoke an AoO from the target you are trying to garrote. If the AoO deals damage, then the garrote fails.
Step 2 Getting the garrote into place
You need to loop the garrote around the victim's head by making a successful melee touch attack, to grab the target. (Unlike a normal touch attack, this does not allow you to ignore the target's armor. If your target's neck is also protected, you might not be able to place the garrote properly.) To determine garrote AC, use its size modiefier plus any of the following special modifiers that apply
Nat AC = AC bonus
Full Plate = +4 AC
Leather collar = +4 AC
Gorget = +10 AC
If you fail to hit with the melee attack, the garrote fails. However, if you have multiple attacks (due to a high BAB), you may attempt again with the lower BAB attacks
STRANGLE
Make a grapple check, if you succeed, you have started to strangle your victim. You immediately deal the garrote's damage for that type. You get to add 1 1/2 x STR mod, for using 2 hands.
If you fail the grapple check, you dont start stranglingor deal damage. Your target slips free and is no longer grappled. You do not automatically lose the grapple check if your target is 2 or more sizes larger than you are.
After you succeed in strangling, move into the target's space. Moving as normal provokes AoO's from threatening enemies, but not from your target.