Pretty sure the reason others use butt joints with a backing block is it's easier to make. It's not in any way superior a scarf is stronger and the wood will bend more naturally after it's scarfed which it most certainly won't do with a backing block.

Put a piece of plastic wrap under the seam so you don't glue the panel to the floor (zip lock bags or other plastic film will do). Deburr the butt ends of the panels by lightly sanding them with fine sand paper. Tack the panel in place with fine wire brad nails, or if you are on a cement floor put some weights on the panel to keep it from moving.
Now cut the Light fiberglass tape that comes in the kit a little longer than each of the panel seams. Mix 1 1/2 oz. epoxy.
Brush some epoxy on the panel seams. Place the glass tape down on the seam and brush epoxy over the tape. Paint some extra epoxy on the wood for 1 1/2" on either side of the tape. Tamp any air bubbles out of the glass tape with the end of a disposable bristle brush. Leave the glass tape and epoxy for several minutes to allow the epoxy to soak into the wood and the fiberglass. The fiberglass should turn completely glass clear. Apply a little more epoxy if you need to, and get the tape completely "wetted" out. Paint a little extra epoxy on the tape and enough on each side of the tape to "float" the tape (so the resin on either side of the tape is the same height as the tape).
Do not proceed until the tape is glass clear.
Your kit comes with a 4" wide strip of mylar (clear plastic). Cut a piece of mylar a little wider than each
of the panels and put it over the glass tape. Roll the mylar into place so that you do not trap air underneath.
If there are small pockets of air under the mylar, use the corner of your squeegee to work the bubbles out. Cut a piece of 3 1/2" wide, flat, scrap plywood (a flat piece of 2x4 will also do; make sure your scrap wood is not larger than the mylar!) and place it on top of the mylar and glass tape and weight it down with several bricks This will ensure that the panel ends forming the seam are pushed flat against the floor
and that the fiberglass tape and epoxy are flattened out into a smooth seam. You want enough epoxy under the mylar so that a little will squeeze out the sides when the weight is applied. The excess epoxy that has squeezed out can be smoothed out with a brush or squeegee. This will reduce the amount of sanding necessary after the epoxy has cured.
Go on and glue the rest of the panels for one side of the boat in a like manner.
Leave the panels to harden. Remove the mylar and save. Using a sharp razor knife, trim the fiberglass
at the butt seam flush with the plank edge. Be careful you don't remove any wood and that you don't pull up on the glass tape as you trim it.
Glue the Other Side of the Panels
Take each of the panels you have just finished, and turn them over carefully. Spread your arms wide and
grasp the panel in the middle. Turn it so that the ends of the panel are supported by resting on the floor as you rotate the middle of the panel. Sand the epoxy that has run through the seam flat. Glass tape this side just as you did the other side. Allow to cure. Trim the fiberglass flush with the panel.
Carefully remove the excess epoxy from the panel edge with a wood file or sand paper.