[SOME SPOILERS FROM THE BRUSHES WITH DEATH DLC]
Rest In One Piece Achievement requires you to find last resting spot for the skull you obtain from Voyta (The Painter), at the end of the DLC.
Required: A spade (shovel). And get The Brushes With Death DLC, and finish it.

To find this resting spot, and anyway to get this achievement. You need to go the grave of Vojtech. You should be familiar of it or where it is by now, but if you are not. For example you forgot, it is totally okay! I will explain how to find it. It should be in the map, south of Miskowitz. If you cannot find it, its okay. Then Head to Miskowitz, a town between Suchdol and Kuttenberg, in the Main Road. In there, head south. Follow the main road. Once you find abandoned Tavern, turn behind it, towards the abandoned/burned Town, called Mezholes. In there, head East. Until you arrive to the forest. (You can check the pics in this guide, to find and see the grave location) Once there, you will see the tree, and under that the grave. Dig the grave, and put the skull there. AND WOLA. You have the achievement.
(After that, i ofc recommend burying the grave back. But thats ofc up to you. Doesnt effect anything)