A scarf isn't difficult to make, though it might intimate a few initially. If the scarf will be under paint, it really doesn't matter how you cut it, so long as the slope is about what you need. You could hack it out with a hatchet, butter it up with thickened epoxy, mash it together and under some putty and paint, no one is going to know of the wood butchery below. I usually employ a power plane to rough in the scarf, which just as easily can be done with a sharp hand plane or even a belt sander. Once it's close, I'll fine tune it with a chisel, hand plane or a sander.
You can use a jig of some sort and I have a couple I've made, but in reality, I find I can hack out a scarf faster using hand or power hand tools, compared to setting up a router jig. I do have a circular saw jig that I use for full width panel scarfs, but these aren't needed very often.
|6:1 scarf with slight notch for brightly finished (top) surfaces|
In order to get a joined plank to accommodate the bends and twists in a hull, it's often necessary to cant the scarf joint a bit, so it better follows the ultimate shape of the boat. This brings up the crooked plank trick. Essentially you're aligning the grain to follow the hull's curves, yet with a flat hunk of wood. With plywood or solid timber, sometimes it's necessary to cock the adjoining piece to do this.
There's no special trick to this, just align the scarf or butt joint to move the grain in the direction of the curve. On the workbench this will look weird, but once bent around the boat, it'll look normal.