Position baseboard against the wall, and use a level to check if it is level at the top and flush with the floor (Image 1). If not, scribing is needed.
Wedge the molding so that it is completely level — check this with the level (Image 2). Use a temporary fastener to hold it if required.
Cut a small piece of wood that is slightly taller than the biggest gap between the molding and the floor. You can use this as a scribing block. Place a pencil on top of it, and slide the block and pencil across the floor surface, so that the pencil marks a guide line on the molding matching the profile of the floor (Image 3).
Use a jigsaw to cut away the excess molding, following the pencil line carefully (Image 4).
Fix the length of molding in place. It should be flush with the floor and level along its length (Image 5).