How do you make a straight taper over a long strip of balsa (or spruce, basswood, etc.)?

For example, a 1/8" thick x 20" long strip, tapering from 1/2" wide at one end to 1/4" wide at the other end.

Thanks all

I would use a long straight edge to mark the cut line and then cut the wood using a scroll saw or equivalent. Then sand the tapered edge with a long sanding bar. This assumes that the strip stock is tapered in only one dimension.

If what you are asking is related more to a two dimensional taper such as an aileron which diminishes in width from one end to the other but which is also tapered from leading to trailing edge, you would follow the same procedure as above but you would then have to mark the trailing edge of the aileron so that you know how much material to remove from each side to achieve the desired taper in this dimension and start sanding using a sanding block. I like to use a dark marker pen to mark the leading edge of the aileron so that I do not inadvertently sand the leading edge too thin while trying to get my taper.