How to draw FBD of earth?

Newton's law of gravity tells us that if the gravitational attraction of body A acting on body B is $vec F$ then the gravitational attraction of body B acting on body A is $-vec F$. In other words, it has the same magnitude but the opposite direction.

So if the Earth attracts the block with a force $mg$, the block also attracts the Earth with a force $mg$ in the opposite direction. When you throw a ball in the air, the ball attracts the earth with the same force that the Earth attracts the ball. But the mass of the Earth is so large that its acceleration is negligible, so usually we can safely ignore the motion of the Earth.

If the block is standing on the ground then a free-body diagram of the Earth (showing only the forces due to the block) would show an upwards force $mg$ due to the gravitational attraction of the block, balanced by a downwards force $N=mg$ which is the normal force that the block exerts on the Earth.