This cheevo will be awarded for completing all story missions in "Episode IV: A new hope"
This is story related and therefore unmissable.
The story missions are;
1. Boarding Party
2. Hunk of junk
3. Best Leia'd plans
4. This is some rescue
5. Stay on target
No need for a detailed walk-through of each level as its fairly simple and linear, plus there are plenty of marker points to the next objective.