First you can gently remove any stones or parts of stones left in the area.
How to replace siding with stone veneer.
Apply mortar and attach stones once the lath is attached apply mortar to the lath 1 16 to 1 8 thick.
Then apply mortar to the stone and attach to the surface.
You can use the stiff metal brush to clean the exposed area of wall as well as the stone that has fallen out.
Angle iron is also necessary above doors and windows to support the brick directly above.
According to the same source the mortar used on the back of the replacement stone veneer should be applied with a pointed trowel.
Using your wheelbarrow and buckets to mix the concrete then apply the mortar to the exposed area on the siding in a thin layer of about 1 8 inch.
Carefully remove the rest of the stone that remains in the area as it s easiest to start laying stones from scratch.
Either the same stone can be put back or a new matching stone can be used.
If stones or bricks have fallen off the siding they need to be replaced.