This shows how the burnt brown edge shows the taper shape.
The closest edges has been sanded, the fin will be flipped over and the other leading edges tapered.
Be sure the width of the brown edge is consistent.
On the left - the outside edge of the Main Fin with the burnt edge (and hold down tab) before sanding.
On the right - after sanding the outside edge.
Don’t sand off too much or change the shape or size of the fin.
Just sand enough of the burnt edge to smooth off the hold down tab for a better gluing area of the cardstock Fin Tip piece.