How do I make a mirrored part of a mesh merge with the one I modeled I created this head by mirroring the right side.  I didn't know how to merge the mesh but when I checked the Merge option in the Mirror Modifier, as I was prompted when I typed the title, I was able to grab all of the mesh.  Then I could mark the seams for  the UV map.  When I unwrapped the mesh as a UV map, it appears that only half of the mesh unwrapped.  I think I made the mirrored portion of the mesh of the head a part of the whole so what did I miss, and how can I fix the unwrapped mesh?
If you apply the mirror modifier after UV unwrapping, you have two identical UV overlapped.
Select linked faces (mouse over the UV, then press L) and move the UV to reveal the other UV.

To mirror the UV horizontally, select the U option before applying the modifier