Depending on whether it is the original, you may have two levers, either hand or foot. If you have two levers, one will be to fill the bowl, the other will be to open the valve and dump the bowl. If the contents are liquid only, you would just need to dump the bowl. Add water to dump solids. If only one valve, you would move the lever partway to fill, and all the way to dump.
It would be a very good idea to put some plain water in it first, to make sure it works as intended, and the dump valve will hold the contents. Also good to make sure the dump valve will open, and that you have the proper adapter for the slinky before filling the holding tank with waste.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.