The technique for installing a fireplace surround is straightforward. If you are replacing a surround for a working fireplace, then you need to make sure that it is compatible with your fire type — check the manufacturer's instructions. If you are working on an old fireplace, you need to follow all the guidelines and make sure that your local building inspector has been informed.
Apply white mortar to the legs. An adhesive spreader is the ideal tool to texture the surface of the mortar to increase grip. Lay a drop cloth to protect the hearth.
Position the upright, using a level to get it plumb. Don’t worry about getting mortar on the wall — it will be covered by the surround — but make sure you keep the marble clean. Check that the two legs are level, then allow the mortar to set.