The Symmetry mode in Space Engineers is for making a symmetrical ship whilst building it.
The Mirror feature in SEToolbox is for making a symmetrical ship after building it.

Due to whatever factors, you may have forgotten to use Symmetry mode in game, or you need to rebalance a ship that is frankly a little uneven, or you've decided to replicate an ship structure within the same ship.

The following are steps to making a symmetrical import (after the fact), or for fixing what Symmetry mode could not.
Basics: You slice the ship in half and delete one side, before using the Mirror function in SEToolbox to create a 100% symmetrical ship as follows.

1. Import the ship using SEToolbox (using whatever settings you are comfortable with).

2. Open in Space Engineers, and marvel at how great it looks, and how unsymmetrical it appears.

3. Slice through the center of the ship, taking off the half you find ugly. If you can't decide, just flip a coin.

4. Make sure you have cut all the way through. double check.

5. Delete the side you side you don't want.

6. Create some nice photos of this moment. (Here's what the inside looks like everyone!)

7. Set an even 'symmetry' plane (you may have to add an extra block here to get the plane in the correct place), using the 'M' and 'N' keys.

8. Or set and odd 'symmetry' plane (just remember to not delete the centreline cubes), using the 'M' and 'N' keys.

9. Save and Exit.

10. Load the 'world' in SEToolbox, and apply mirroring!

11. Reload in Space Engineers, and you are done!

Last edited Jul 10, 2014 at 11:07 PM by midspace, version 3