I use this step by step procedure to shape L.E.
1) a line must be drawn on the L.E which indicates the linear axis of the ribs,note this line is not necessarily the symetric line of the balsa spar used for L.E. but must join the symetric line of the wing's profile. this line guides you while carving the L.E. to know if you are carving toomuch or not.
2) to prevent any erosion on the skin caused by cutter I simply use a piece of tape at the tip of the blade to make a safe end blade.
3) L.E. is carved while the face of the blade is laid back on the skin. it's important to touch the skin with the face of the blade otherwise you will over carve the L.E.
4) a piece of tape is attached to the skinapproximately 1/8'' far from the L.E. this protects the skin from over sanding. thenL.E. must be sanded along with the the skin while sand paper touches the tape attached to the skin.
5) tapes are now removed and the rest of the L.E. is sanded equal from top and bottom of the wing regarding the center line drawn at the first step.

Ehsan