ok, you see the soft plastic bit in the loop? Push that down in the loop from the narrow end to the wider part, this should releast the plastic part.
Now slip the top of the stocking (usually a lacey bit) and put it between the loop and the plastic part, push the plastic part back into the wide part of the loop and push up into the narrow part to secure. The stocking should now be held in place.
A little tip: lengthen the suspender straps as far as possible to attach to stockings while you have them on and then tighten to comfort....this really helps when doing the back ones!!
I hope that made sense???