This is so simple that I hardly think it needs a tutorial, but I offered, and was reminded, so here it goes.
So, you want to connect something like a case fan or some LEDs and you need to find 12V somewhere. If there's a nearby drive with a molex, then it's easy.
First unplug the power connector from the drive.
Then, strip your new wires by about 1/2"
Next, after twisting the inner strands together, poke it through the female connector in the molex from the back, so that it sticks out a little.
I've removed the insert from the socket here to show what I mean, but you don't need to.
Then, take a tiny screwdriver or something and fold back the bit of wire which was sticking out, making sure that all the strands are inside the plastic housing.
See the left hand one here, upper left area you can see the new wire
Then plug it back into the drive and you're done.
From the back it looks like this:
Which I think you'll agree, looks neat enough.
Obviously it can't be done with really thick wire, but you shouldn't need to use that anyway.
Told you it was simple!