muffaWhile in game:
F10 (menu) -> Settings -> Interface -> Commands
Make sure the following two are checked:
Area Mex
Area Terra Mex
Now select a constructor or commander. On their commnad card there should be a button for Area Terra Mex (shortcut ALT+W).
Click and drag to expand a circle and select at least one metal spot. The con should walk to it, build it and make a small wall around it.
Just be careful issuing this command to a group of cons. If there are too many, they'll push each other onto the terra area and will trap themselves building the wall around the mex. This shouldn't happen if you only have 2-3 selected.
To build something on a platform of the exact size of the building, let's say a Gauss (but it could be anything):
Select a con
Press C (defense tab)
Press T (gauss turret)
Before placing the turret, move the mouse where you want it to be, hold ALT and use mousewheel to adjust the height
The con will terra the necessary tower (or dig) and then make the building
No need to save presets for single buildings. There's a shortcut for it somehow.