Read your stove manual to determine the size the hearth pad needs to be. All stove manuals should state the required clearance space around the stove; the typical clearance is around 18 inches or more.
Cut a piece of 3/4-inch thick plywood to the size and shape you want your hearth pad to be using a saw. Cut two identical-sized pieces of ceramic backer board, using a utility knife to score along the cut lines and then snapping the backer board along the score line.
Place the plywood in position on the floor where you want to install the hearth pad. Use 2-inch galvanized screws, driven every 5-inches around the perimeter into the floor to secure the plywood. Repeat with the two sheets of backer board. This is the base for your brick hearth pad.
Mix a batch of mortar in a bucket, following the mortar manufacturer's mixing instructions. Use a trowel to lay a line of 1/4-inch-thick mortar around two-bricks wide on top of the backer board. Run the tip of the trowel through the mortar to furrow it and then place a brick into the mortar. Tap the brick down into the mortar with the butt of the trowel.
Spread mortar onto the end of another brick and lay it into the mortar, abutting the mortar-ended brick with the first brick laid. Continue laying bricks across the top of the backer board and the sides as well. If you need to cut a brick to make it fit the design, place a brick chisel onto the cutting point, and tap it forcefully with the mallet to cut the brick.
Check that the bricks are flat using a level. Once the hearth pad is laid, but before the mortar is set, use the tip of the trowel to smooth over all the mortar lines between the bricks, scraping away any excess.
Leave the hearth pad to cure for the amount of time specified on the mortar packaging; then you can install your stove.