The correct series of steps to demonstrate conservation of momentum in a system of two moving particles that collide would be:
Determine the momentums of the two particles before the collision and add them together. Determine the momentums of the two particles after the collision and add them together. Verify that both sums are the same.
This reflects the principle of conservation of momentum, which states that the total momentum of a closed system remains constant if no external forces act on it. Thus, the total momentum before the collision should equal the total momentum after the collision.