Preparation all depends on the lady. Some have wanted me to prepare them using a butt plug and lube which I enjoyed, another did her own BP with no intervention from me.
My most recent experience of 'A", the lady just needed some lube and a couple of my fingers and she was ready to go. Interestingly, this WG also said that anal is better early on, first pop usually, as she reckoned on round two that guys were seldom hard enough to make penetration. Have to admit that I think she may have a point.
There are also WGs who willingly take anal from all comers, those that will if comfortable, and those who say at dircretion (marketing ploy) yet rarely ever do.
Either way, good lube helps.