This tutorial will teach you how to delete the smooth proxy and then mirror your geometry into one solid model.
Step one - First we're going to have to open the outliner. To open the outliner go to window>outliner.
Step two - Select the group (it'll be named something like smoothproxygroup). Go to edit>ungroup. This will ungroup the proxy group.
Step three - Select the proxy object and delete it.
Step four - We need to select all the border edges. To do, on the polygonal menu set, go to select>select border edge tool. Now double click on one of the border edges. It doesn't matter what border edge you double click on.
Step five - The reason that we need to select all of the border edges is we need to flatten it. This is because when we mirror the new side, it has to have a flat surface. Otherwise the merge threshold will be messed up. As you can see our fish model is not flat along the edge that the new side will be connected to (this may not be the same on your model. Most likely it won't be this obvious).
To fix this problem go to the scale tool and scale it along the X axis until the scale handle stops moving.
The other way to fix this is using a MEL script. Paste this script into the command line. scale -r -p 0cm 0cm 0cm 0.0 ; This will flatten the edge.
Step six - These edges may not be directly in the center of the scene. To do this turn on snap to grid. And use the move tool and snap it to the center line.
Step seven - Select your model and go to mesh>mirror geometry. Click on the square beside the text so you can open up the options. Set the mirror direction to the axis that you want to mirror your model across on.
Step eight - Open the attributes editor by going to window>attributes editor. The polymirror tab should already be open. As you can see in the image above, the opposite side didn't mirror correctly. This is because the pivot isn't lined up right. To fix this, set the first pivot attribute to 0.
Now, as you can see that as you zoom into the model, some of the vertices have been merged that we didn't want to merge.
This is because the threshold is too large. Set the threshold attribute in the attributes editor to 0.010. This should work for most models, but if your mode has more detail, this should be even smaller.
Step nine - Now, to smooth your whole model, go to mesh>smooth.