I would suggest dipping them in Starsan before you cut them, but after you take off the stems.
And instead of quartering I would suggest mashing them, and then putting it in a bag. Because the reason you mash them is to increase the surface area contact between the cherries and the beer.
As for when to add I would agree with nsrooen, add them in the secondary. Or if you don't plan on using a secondary, after the primary fermentation stops, then add them in, and let them sit for about 5 days.
As for amount: I would go with 1-2 lbs. Or 3 lbs if you want it to be IN YOUR FACE cherry flavor.