Decide how large you want your mantel to be, taking into consideration the room's size and proportions. Outline the dimensions in painter's tape.
Determine how far you want the stone mantel to extend. Mark its footprint on the floor in masking tape.
Cut your metal lath sheets to dimensions that will fit inside the marked-off area inside the painter's tape, both against the wall and on the floor. This will act as a base for the mortar, allowing for a sturdier and more durable fireplace.
Nail the metal lath to the wall and floor. Remove the painter's tape.
Mix your mortar. Start at the bottom left corner of your mantel. Apply mortar to the stone you want to go in that spot, and then apply the stone.
Apply a stone to the bottom right corner of the mantel. This will give you two points of reference as you apply the first row of stones.
Apply a generous layer of mortar with your trowel as you lay each stone. Apply mortar in-between the stones. Smooth the mortar in-between the stones with your hand.
Apply the stones in horizontal rows, always starting at the wall moving from one corner to another. This is a building pattern that will keep your fireplace structure stable.
Wipe any excess mortar off with a damp cloth. If the metal lath is visible at the edges, apply a layer of grout to cover it. Allow the mortar to dry for at least 48 hours.