The first stage is when the nebula collapses and begins to rotate.
The second part is the planetary disk. The planetary disk was formed when the nebula flattened. The sun was formed in the middle of the disk from all of the heat.
The third part is when particles start to join together and form clumps of matter. Gravity caused the clumps to collide, and over time the collided clumps expanded to form planets and asteroids. This stage is called accretion and planetesimal.
The last part is cooling. After all the heat and collision the new planets start to cool and become what there are today. The great Milky Way Galaxy!!