The mountain climbers figure 8 knot is where you tie a loose figure eight with a long tail, then you run the long tail through the eye of the clew and then trace the tail following the exact path of the figure eight back around again. So you basically have two figure eight in parallel. The normal figure eight that is used as a stopper knot will not hold and can shake loose at any time (which is why it makes very bad stopper knot). The climbers figure eight is different from the normal figure eight (in that it is doubled).
Look online, you should be able to find the figure eight used by climber and you will see that it ends up being rather big, but the advantage is that it will never shake loose (that's why it is used by climbers). I use it one my climbing gear when going up the mast with an ascender. I also use if for the lines
that hoist the dink up at night.