THE SHELF:

Applying thinset to the top of the header
prior to placing the shelf
 

Filler panels, if any, are generally installed using only thinset mortar. Use ¼” spacers on all sides meeting other stone. With prefabricated fireboxes, the black flashing on all sides of the firebox opening can normally be covered with stone, but take care not to cover louvers, glass, or other access points.

Thinset does not hold well when applied to metal. If the filler panels are to be applied to metal only, use a construction adhesive (such as “Liquid Nails”) instead of thinset mortar. 

Installing a bottom filler panel. 
(Note that the spacers have been cut in half.)
 

Some installations will not have side filler panels.  Temporary supports may be used to keep the top filler panel in place while the thinset mortar cures.  Another method of installing a top filler panel without side panel support is shown below:

First, screws are driven into studs

Next, the heads are ground off

The back of the filler panel is marked

Slots are ground where marked

Thinset is applied (Also, note the spacers.)

 
The unsupported upper filler panel has been
installed and is being grouted.

Caution:
Be careful not to attach any stone to metal which will heat up when a fire is started. Thermal expansion of the metal may break the joint and possibly the stone. Be careful when drilling through any portion of a prefabricated firebox. Although the flashing on some places on some models may be penetrated, this could cause hazardous conditions in other places or on other models.

GROUTING:

Grout and point all the joints. (Caulk is often used in the joints between the mantel and the wall to prevent cracking caused by wall movement.) Sponge all exposed surfaces with clean water as the grouting operation proceeds to ensure that mortar does not dry on the mantel face.

Applying grout to the mantel joints.

Applying grout behind the lower filler panel

Finished mantel installation