Summarize:

Unscrew the masonry screws and take the wood down. Most likely, you will need to cut the tile or tiles you'll be placing at the bottom of each leg to make them fit. Taking into account the height of the legs (the non tiled areas to the sides of the firebox), the height of the tiles you are using, and the width of your grout lines, you'll need to make an estimate of how much you'll need to cut off the bottom tile. For example, imagine your legs are 37 inches tall. If your tiles are four inches tall, and your grout lines are 1/4 of an inch, this means that each row will be four and 1/4 inches tall. Eight rows of tiles would be 34 inches tall, which is not enough to fill the space, but nine rows of tiles would be 38 and 1/4 inches, which is too tall. Based on this, you know you'll need 8 rows of tiles, with a 3 inch space to fill at the bottom with a cut down tile or tiles. Trim your 1X3 piece of wood down to the height of the estimated space (e.g. 3 inches in the example above) and place it across the front of your fireplace, at the bottom, up against both legs. Check to ensure that it is level, and secure it in place with masonry screws. If you plan to lay a row of tile insides of legs, you'll need to cut a small piece of wood of the same height to place inside the legs. Mix up some more thinset, and then, using the same method as you used for the upper field, apply tiles above the support ledge and work your way upward. If you did your math right, you should fine that the tiles line up perfectly with the upper field, with room for a grout line.  As before, use spacers to keep the distance between your rows even. After you're done, allow a few hours for the tiles to set and then remove the support ledge. You'll need to calculate how much to cut off each tile you'll be applying to the bottom of the legs. You already know how much space is left, but you'll need to account for a two grout lines as well (top and bottom). Measure and cut your tiles with a wet-cut tile saw. Apply thinset mixture to the back of a cut tile using the edge of a trowel. Gently push the tile in place and adjust until evenly positioned. Repeat this process along the bottom of the surround. Allow several hours for the tiles to set.
Remove the support ledge. Estimate the cut. Cut a new support ledge. Work upward. Cut your tiles. Lay the last tiles.