A moldering privy requires a "bulking agent" to properly decompose/compost the human waste for which the privy is built.  In most forest situations, this will be forest duff - leaves and twigs that have started their own decomposition process along with the other bits of moss, dirt, and bugs that come along with them.   In other circumstances, this might be bark, wood shavings, or garden mulch.  These latter items can be purchased commercially for use where natural forest duff isn't available.

After each use, a handful or two of the bulking should be sprinkled over your 'deposit.'  This not only helps the composting process, but it is key to keeping the odor to a minimum. 

Adding too much bulking agent isn't a problem to the composting process, but it does fill the holding compartment more rapidly, and require more frequent acquisition of more bulking agent (and expense if commercially acquired.)  So try to use enough to cover the deposit, but try not to use too much.  And if you accidentally put too much in, don't worry about it.