I recommend saying it before you enter the confessional. It help to rehearse not only the words, but also the thoughts and motivations that the words are supposed to drum up in you. I suggest saying it slowly while you are waiting, meditating briefly on what it *really *means to be sorry *because *“they offend thee my God”, or what is *really *involved in firmly resolving something.
The act of contrition in the confessional immediately before absolution is mandatory, because you can’t be absolved unless you manifest contrition anyway. Hence the absolution would be invalid and illicit without your act.
There is an exception: if you are physically unable to express it, like if you are in a coma and about to die, then the assumption is made that you would if you could.
Who told you to say it *after *absolution? That doesn’t make sense.
The priest’s clothes don’t matter, but the usual equipment is a purple stole that signifies the penitential work he is doing. In face-to-face confessions, sometimes he has a crucifix in his hands that he presents to you to kiss out of sorrow for your sins.
(By the way, “vestments” technically refers to the robes and so on that he wears for mass; the collar and the other black clothes he wears in everyday life are usually called “clericals”. And a priest can administer any sacrament regardless of how he may be dressed at the moment - e.g. military chaplains hear battlefield confessions in fatigues; and I have heard one story of some bishops, wearing lay clothing, consecrating another bishop, at a house party, secretly in the kitchen, for fear of the communist authorities!)