Making a scarf joint is a boatbuilding skill. It's something that no one starts out knowing how to do, but a lot of people are able to learn how to do it relatively quickly. It's not that hard to do with a sharp block plane. When you're done you'll be standing in a pile of wood shavings, feeling like a boatbuilder.

I think it's usually easier to cut the panels to shape and then scarf the ends, than to try and scarf uncut plywood panels.

You don't need an elaborate jig. If you're using 4mm ply and want an 8:1 scarf, the joint length will be 32mm, about 1 1/4 inch. Draw a line parallel to the end of the ply 32mm in from the end on both pieces to be joined.

The big step is figuring out which piece gets cut on which side. The best way is to hold the pieces in the position they will be in the finished boat.

Lay the first piece on a workbench with the end of the wood exactly on the edge of the work surface. Lay the second piece on top of the first piece with the end exactly on the 32mm line on the first piece. Make sure the 32mm line on the second piece is visible. If you lay your block plane across the edges of the two pieces of plywood it will assume the proper angle. Now you just plane back until your beveled surface reaches from the edge of the workbench to the 32mm line on th top piece. Since you're working plywood the bands of ply will be exposed one-by-one, giving you a guide as to which parts of the cut need more work.

It doesn't take long to bevel 4mm ply and you make mistakes much more slowly than with a router or a belt sander.