I'm your expert 
. But not sure I can explain well.
Ideally, you would wrap it rightly and completely in something like bubble wrap (or several layers of plastic bags), just to protect the surface.
Then it goes into a fairly generous box...
Stuff into the box all around the item those foam/peanutshape shells (or something equivalent, like cardboard pieces or excess plastic packaging recycled from other purposes (see below))), possibly lining the inside of box with yet more pieces of cardboard.
Tape it all up, you should be able to shake the box and NOTHING inside moves. See if you feel confident dropping the lot on the ground from waist height.
If not, then take the whole first box (still taped up) and put it inside a still larger box, again lining (stuffing) every inside space, all around the inner box, with either cardboard of pieces of plastic packaging (like the shaped plastic trays out of biscuit packs). Tape it up again, and see if you feel inside movement, or confident about it being thrown about. Double boxing probably not required for your item, but depends how fragile you think it is; double boxing is often the only way to ship glass safely.
If you've any doubts, try not to send using Parcels but using 1st or 2nd class; parcels are treated the hardest. Alternatively, courier'd items are treated very much better than RM Parcels, but courier is never economical for me.