Use a longer handled chisel. First cut, at the saw cut line and tilt the chisel toward the saw cut and cut the full length of the notch so the width of the cut starts very narrow and widens to only about 1/4" at the end of the notch. Subsequent cuts starting toward the end of the notch and work back toward the pin line with each subsequent cut flattening the chisel gradually until the last stroke when the chisel should be flat as you make the cut at the pin line and you're just removing a very thin layer the length of the notch.
I use Japanese chisels and hone them frequently.
Good question. I had to think about this. The first cut described there's no scoop. The following cuts that start out at the end of the notch there's also very little scoop-probably none at the first cut. As you work back toward the pin line there's progressively more scooping and the final cut that bisects the pin line has the most scooping. All done with the flat side of the chisel down.
Re Chris's comment, I don't lengthen the chisel, I have several chisels of varying length and width. I find the longer chisels are easier for that section and because of the length of the notch the longer chisels also prevents me busting my knuckles on the edge of the bridge. I use shorter chisels for the treble section and bass bridge I also tend to lay out bass bridges with the pin line perpendicular to the strings not a continuous bevel that produces non equal speaking lengths. Not convinced you can hear the difference but I do it anyway, usually.
Quality chisels are so important for this and I've found nothing better then the Japanese chisels. Good ones are quite expensive but worth it.
Of course you can also make a bridge notcher as has been discussed by Del and also Ron Nossaman. I've seen one in action at the Charles Walter factory. Very nice and very fast.