Basically is using the Betta Box and Mirror Method.
I normally will place the angel in the betta box to let the existing angels get used to the the new angel... Up to a month depending on the situation and it will allow me to pump up the new angel so that it have better chance of fending off the chasing when introduce..
After you introduce the angel, put a mirror beside you tank, so that the angels will be distracted by their own image than to chase the new angel... This will carry on up to 1 week.. Depending on the aggression level, so have to moniter daily..