Sorry in advance for what may seem like rudimentary questions...but
just want to make sure i am doing this right.

I am tapping into an existing outlet to add a new outlet about 6 ft
away. The location is around a corner wall. So in my drawing below,
o1 is the original outlet, x looks like a junction area in the wall
that provides the power. o2 is where i want my new outlet to be
located.

-----o2---------------------
| |
x |
| |
| |
| |
o1 |
| |
-----------------------------

My plan is to splice the wires from X and run to o2 using #12 or #14
cables. I wanted to cover this up using wire molding so it looks neat.
My question is, how do you install these things. Do you run the wire
through them first or do you install the wire molding first then run
the wire through them? The wire wolding i have seems to have a long
metal sheet as the back plate to which the molding itself is attached.
That metal sheet can slide in our out (left or right), but i don't
think you can snap the moulding on to that...implying that i have to
'feed' the wire through the molding and therefore must run the wire
through first before attaching these molding.

I did a google and could not find much on the proper way to install
these things and the instructions are not very clear in terms of what
you do with the wire and when.

thanks for any advice.