This is taken straight from the priest.mdl
GeosetAnimId 0,
}
Bone "Plane01" {
ObjectId 17,
Parent 15, // "Staff"
Billboarded, <-----billboard command
GeosetId 0,
GeosetAnimId 0,
}
As you can see, it's quite simple to do: you just have to add the 'Billboarded,' line to a bone. NOTE: this will billboard all the polys related to it; so in some cases you may want to create a new bone, attach it to an existent one and then geomerge a face on it.