I like to stack and scarf 4 panels at a time. Stacking helps setup the angle and keeps the joints more consistent than doing them one at a time.

I use a regular plane for the main wood removal and fine tune with 80 grit sandpaper on a flat wooden block.

I mark both the top and bottom of the panels so that I have reference lines for alignment later. My best joints have been made by stapling the panels to a backing board (with plastic separators of coarse) that way I don't have to worry about panels sliding around while clamping.