top of the piece you are going to install.
With a pencil draw the outline of the baseboard and cut with a coping saw.
If you are working in a mid-wall location the best joint is known as a scarf joint. Basically miter both pieces @ 45 degrees vertically (perpendicular to the floor). Each piece should be a mirror image of the other. The object in both cases is to avoid a crack which can be looked into directly. I could draw you a picture better than I can describe.