Scarfing plywood is easy and fun once you get the hang of it. The idea is to create an 8:1 slope on the two pieces and then glue them together. So, a 3/8" piece of ply would get a 3" scarf. To do it, just stagger the two pieces. I think it works best if you put all the pieces you're going to scarf together and add another on the top and bottom. Stagger each one back the 3" and then go at it with a power planer and finish off with a disk grinder checking for flatness with a staight edge.

To get nice layups, get your scarfs as flat as you can and then screw one panel down to a large flat surface (like another piece of ply), then pre-fit your second piece and mark it and actually drill the screw holes. Then you won't be guessing when you're glueing them together and things are getting messy

See this post for some good photos of this kind of setup. There is also a good article linked from this post, that shows you how to create a scarfing jig. Using a 10" skillsaw, you can quickly make 3" scarf cuts that are perfect every time. You don't need to do this unless you're building a lot of boats though. The first method works fine for most scarfing projects.

A butt block is fine as long as you don't mind having that extra block of wood. I'd also think twice about using a butt joint if the wood is going to be bent a lot. You can do the fiberglass method mentioned in your post but I don't think that's very strong. If you're laying a couple of sheets of ply onto another surface and there are other structural pieces that will hold them together, that may work fine. Making a scarf joint though will often give you the cleanest and strongest joint.