File the side of the toe plate and the wood together, so you end up with a bevel on the toeplate and no little "step" left on the wood. It should end up being a perfect blend. Then you need to sand those two bottom corners their entire length, so the they are parallel. Otherwise, you will have a narrow part by the toe plate, then it will widen out just past that.
Once the toe plate and wood is filed, you can strike a line with a straight edge against the corner of the toe plate. Do the other side and make sure the two lines are straight and parallel. Sand up to the lines with a sanding block, then ckeck to ensure there are no bulges or inconsistancies in the flow. Keep sanding until everything flows from the buttplate to the triggers. You'll need to roll the sanding block as you sand, to make the sides blend in. Sounds complicated, but it will just come naturally when you start sanding. Bill