mirroring

%| how do i mirror something in blender?

[S] afterwards [X]
or
[S] afterwards [Y]

or [N]
change SizeX to -SizeX
or SizeY to -SizeY
or SizeZ to -SizeZ

do the

ctrl[A]

(apply …)
after, to get faces/normals right

ctrl-a?

I always select all vertices of the mirrored object and press ctrl-n to set the normals to outside…

btw, if you want to model one side of the model and see the other side mirrored (for instance if you model a car), you can instance the object with alt-d and then mirror the instance.

Roel