This is a model of a Star Destroyer I found, and I thought I would share it. I was too lazy to add much detail to the control tower, but you can add however many details you want!
Step 1: 1-5
Follow the pictures in order:
1. Fold in half both ways.
2. Mark fold.
3. Fold from the top corner to the mark fold.
4. Fold the flap back to the folded edge.
5. Match the other side.
Step 2: 2-8
6. Unfold everything.
7. Mark fold again, but on the other side.
8. Do everything again on the top half of the paper.
Step 3: 9-12
9. Unfold everything again.
10. Fold the edges on existing creases.
11. Fold the top down on existing creases.
12. Prepare the top flap for a rabbit-ear fold as shown.
Step 4: 13-17
13. Note the reference points for the next fold.
14. Fold the top flap up.
15. Rabbit-ear fold with existing creases.
17. Fold on existing creases to make a triangle shape.
Step 5: 18-20
18. Fold the tip.
19. Reverse-fold the tip inside.
20. Fold the top triangle down.
Step 6: 21-Finish
21. Rabbit-ear fold the triangle to make a tower.
22. Reverse-fold the tower.
23. Flatten the reverse fold.
24. Fashion into a control tower.
Your done! Go force an innocent planet into Imperial submission with your new Star Destroyer!