A baseboard molding forms part of the trim at the bottom of a wall. If you have an external miter that was cracked and need to be repaired, you need to get hold of the following items:
Step 1 - Cutting the Angles
Cut the angles with a miter saw. Position the molding face up and set the saw at a 45 degree angle to the left and cut it. For the other piece, set the saw in the same 45 degree angle to the right and proceed with cutting it.
Step 2 - Repairing the Wall Surface
Remove cracked or loose drywall pieces with the taping knife. Fill any gap that results from the removal with the drywall compound. Smooth the surface with the taping knife and allow to dry. Prime and paint it to match the rest of your wall.
Step 3 - Attaching the Molding
Attach the two pieces of baseboard molding with the nail gun. If there are any gaps, fill it up with caulk. If there are hairline cracks, dig it up with a sharp chisel until you reach the hard part. Fill the gap with the drywall compound and cover it with drywall tape.