First off, I am not sure what you mean about the taper. The wings are built completely straight on the board, as if it was through the fuselage on the airplane. The center of the wing is on the board, while each tip is raised off the board. Is that what you've done? The wing is not completely flat on the board...

As it's pinned down, you're building it upside down. The sheeting you add while it's pinned down is on the BOTTOM of the wing. Laying on the board, you're looking at the BOTTOm of the wing. When you take the wing off and flip it over, that will be the TOP.