You mean how do you move the tombstones to the cemetery?
You need to be in the lot where the sim whose grave you want to move to the cemetery died. You also need a playable sim in that lot.
In play mode, select the urn/gravestone. A message wil pop up with the options "mourn" and "move grave". Select the last option.
Soon a screen will pop up with a list of community lots you can choose from to move the grave to.
When the grave has been moved (ghost will appear and wave goodbye) a message will pop up telling you that the grave has been moved, and you will need to save you game if you want this change to be permanent.
Save your game.
To move the grave to the correct spot in the cemetery, you will then need to go to the cemetery (in build mode, not with a sim), and you will find the gravestone waiting for you by the telephone

Pick it up, and place it in the correct spot. Save the game and you're all done.
HTH.