How do I drop the deck on my craftsman riding lawn mower?

The instructions to do that are in the operators manual if you have one. It is not too complicated and involves loosening some nuts on the linkage holding the mower up at the sides and at the front. If you mean just to lower the deck to mow at a certain level there should be a large lever mounted on the side of the mower with adjusting notches for setting the cutting height.