Oh, that's not a stupid question at all.
I have the headstone plots and urn tables already setup for your sim's tombstones and urns. When you are playing a sim's house and they have a tombstone, or urn, that you want to send to the cemetary just do this; click on the tombstone or urn and select the option to send it (cannot remember the exact term right now). Then a box will come up listing the lots in your neighborhood that you can send it to. Just select the "Finally Free Cemetary" to send it there. You can then save and leave your sim's house and enter the cemetary in build mode to position the tombstone or urn where you want it in the cemetary.