Make a fireplace template. Using a tape measure to gauge the lengths, note the dimensions of the fireplace you plan to build. Using these measurements, create a template on the wood board with painter's tape.
Lay the stones to fit the fireplace template. Place the stones within the boundaries of the template. The stones should be laid as close to each other as possible.
Cut the stones for a good fit. Using a masonry saw, cut the stones in such a way that they fit within the boundaries of the fireplace template.
Mix the mortar in a mortar pan and apply it to the stones. Start filling mortar between the stones using a masonry trowel.
Cut a plywood frame for the hearth. The frame needs to be cut so that it will fit inside the firebox. Saw pieces of 2 by 4 from the lumber. Using masonry nails, nail gun and wood glue, attach the pieces to the bottom of the hearth.
Place and adjust the hearth inside the firebox. Install slate against the hearth frame using the mortar mix. Install the stone on the hearth using the mortar.
Add grout to fill in the gaps between the stones. Use a grout float to add grout into the gaps. Wipe away the excess grout using a damp cloth.
Apply sealer between the slate pieces. Allow mortar to dry for a minimum of two days. The sealer must be applied between the slate pieces once the mortar has dried completely.