Fireplace hearths are typically installed using ceramic or stone tile, such as slate or granite. The basic premise of all good tiling jobs is starting with an even substrate, so you will need to get your uneven floor evened out before installing a fireplace hearth. Thankfully, most minor dips and uneven patches can be levelled out quite simply. Structural unevenness will need to be taken care of by a builder, as there may be more at stake than just a crooked hearth.
Check the hearth area to determine the extent of the unevenness. If there are simply pits and bumps in the concrete substrate or slight warps in the plywood substrate, you can level it yourself. If the whole floor is on a slant, then there might be something structurally wrong with your house. If this is the case, call a professional to assess the situation.
Mark out the area of your hearth with a pencil and a ruler. Mix your thin set mortar according to the manufacturer's instructions. Apply thin set mortar to the dips in the floor to bring it to a roughly level surface. Check with the level. You may need to carefully chisel away at concrete lumps that are really poking up above the general height. Let the mortar set a little bit.
Cut a piece of ceramic tile backer board the size of the hearth. Place it over the thin set mortar and check with the level. Tap down any areas that are still sitting too high, and add a little more thin set mortar under those places which are still too low. Secure the backer board in place with nails every 6 inches in the centre. Let it dry for 24 hours.
Divide the hearth space into four quadrants with a pencil and a ruler; there should be a horizontal and vertical line that cross in the middle of the hearth.
Dry fit your tiles starting from the centre cross point working your way out; space with tile spacers. Mark any tiles that need to be cut.
Replace the dry fit onto the floor next to the hearth so that you have your tile pattern ready to be set in place.
Mix another batch of thin set mortar. Apply a small section in the centre of the hearth. Set down a 1/8-inch layer of mortar then furrow the surface with the notched side of the trowel. Position your tiles as you did for the dry fit, setting them into the mortar by gently tapping them down; space with tile spacers.
Continue applying small sections of thin set mortar and setting the tiles in place. Work from the centre out until you only have the tiles needing to be cut left.
Put on your safety goggles and cut the tiles with the tile cutters where you marked them. Butter the back of each tile and set it in its place individually. Let cure for 24 hours.
Mix the grout as per the manufacturer's instructions. Apply grout across the tile joints after taking out the tile spacers. Use the grout float to sweep the grout into the joints. Do not apply grout to the outline of the tile hearth and the rest of the floor.
Wipe off the excess with the sponge and let dry for 30 minutes. Wipe off the remaining grout haze. Apply a line of caulk around the edge of the hearth. Smooth this over with a wet finger. Let cure for 24 hours.
Clean the tiled hearth thoroughly with the sponge after 24 hours. Do not light a fire in the fireplace for at least three days after installing the hearth.