I know this makes me sound like a total noob (I am), but “Blender for Dummies” doesn’t explain this procedure in a way I can understand (assuming it’s explaining it at all). I’ve constructed a number of towers with shelves, and deleted the first two towers as they were first tries that had fundamental problems. Each tower is a slightly different shape, but each one has its mirror opposite on the other side of the area. The easiest way for me to replace the two deleted towers would be to copy their opposites, move those into place (I did this part already) and then make mirror copies of them. I understand that this is a common procedure when sculpting a complex symmetrical object, such as an animal. Could someone explain, step-by-step, how it is done? Thanks!
Well, that’s quite simple. Once you have your duplicate, go to Edit mode, select all and type:
S, X, -1, [ENTER], [CTRL N].
X, Y or Z, depending around which axis you want to mirror. In plain English: Multiply the size on one axis by -1.
And the [CTRL N] is to recalculate the normals which may become inverted in the process.
Well, assuming the towers are really separate objects, you can duplicate the objects in object mode (shift d). After positioning the copy you can switch into edit mode on the copy. In the toolbar you find the entry “mesh”. There you find an option to mirror your mesh. Play with the axis until you get it right. I needed several tries for the screenshot 
Another option would be to model it using the mirror modifier.
Thanks, folks! I’ll give it a try when I’m able to again. It looks a little like the “Dummies” instruction, but more step-by-step. I must be getting senile; I can’t seem to memorize the steps they leave out, or tell if something’s missing until I reach an impasse. It would be nice if they included a sort of “cheat sheet” in the indices for certain operations; but with such a versatile program, I’m not sure how one would arrange the hierarchy so that the instruction could be looked up.
In case of panic, break the spacebar… 
Well, press the spacebar and type what you want to do. You have to speak Blender’s slang because its vocabulary is limited but it can help sometimes. In the case of the Mirror, you would have find it right away.
I always forget about the spacebar which I seldom use. The middle name of my spacebar is Google. 