Using a batten, the bottom panels are planed to the line right up to the transition joint. The frame angles guide the angle of the cuts along the way.
Battens also hold the bottom planks from bouncing around while you plane. A good trick is to tack on temporary butt joints between bottom sheets with a couple of drywall screws (I use them temporarily everywhere). Note that the key strategy to this building method is lamination, not scarfs or butt blocks. It looks nicer both inside and out and is strong if you put enough layers in.
Getting out the side panels means you have to have the bottom shape at least close to where it's going to be in the end. I sprung a 1x4 spruce plank down each side of the bottom to hold the sheets fair. I previously marked and cut the side panels to the rough shape of the station frame corners. I leave about a half inch because there's always a place that isn't quite right. Then I tacked on the side panels and started trimming. Depending on my mood mostly, I use a plane, a router, a draw knife, and even a spokeshave. The router does most of the work pretty quickly, but the plane is so much more relaxing...
There's no problem getting the angles right at this step. Since the bottom went on first, the angle from the bottom panel just extends across the edge of the side plank. Again, same as you'd do for a chine log, -only easier because you have a guide between frames as well.
I put the transition joint between the first and second frames. A transition joint is a transition from an overlap to a butt joint. Since the bottom and sides are exactly on top of each other at the stem, the overlap gets too weird. Things get weird in the bow no matter what. Even with a butt joint, you don't see the correct chine edge until all of the layers are on, -if the bottom is thicker than the sides. The final bottom sheet will change the line aft of the transition joint, but not much forward of it.
You can see the tumblehome in the stern in this set of pictures. The ply bends easily, -just tie a rope around the boat to hold it in while you drive the screws.