Lining up vinyl pieces relative to the edge of a sign, or to other pieces is possible by simply creating some registration pieces. I use triangles.
In this example, I’d lay the black artwork down first so that it’s square with the sign piece. Then the red artwork (and the triangle that goes with it) would go on the bottom left corner.
After the two colors of vinyl are positioned, all triangular registration pieces are peeled away.
Click on the image below to see an animation.