Lay out all stones, separating corners from flat pieces. This will also help you separate them by size and begin to formulate design aspects to your fireplace. If you're doing this indoors, you may want to protect the floors with drop cloths.
The mortar mix is two parts sand to one part mortar. Mix the sand and mortar first, then add the bonding agent, and finally add water. Follow the manufacturer's directions when mixing mortar, but add enough water to achieve the proper consistency so that the mix is neither too dry nor too soupy. Let the mix sit for five minutes before using it to ensure that it's properly coagulated. When mixing mortar, it's best to pull the bulk of the ingredients to one side in the wheelbarrow or mixing container. This will save you some strokes, make mixing easier and lighten your workload.
First moisten the area you wish to apply stone to (don't moisten too much as the water may evaporate). Apply about a 1/2-inch-thick layer of the mortar mix to the back of the stone. Spread it evenly and cut the edges back with your trowel to prevent the mortar from oozing. Apply the stone to the moistened surface and hold it in place firmly for 20 to 30 seconds on the wall to allow the bonding agent to activate. Begin laying stone at the corners, and work your way inward. If you're working with a partner, it's a good idea to switch sides at some point during the process of applying stones. Everyone sets stone in a different way; switching off helps provide variation to the overall pattern, allowing you to blend the two styles together in an eye-pleasing manner. Conversely, if each of the two people works separately on only one half, the two styles may be discernible and visually distracting in the finished piece.
Once you've set all of the stone, go back and fill in the joints with mortar to prevent the lath from showing through; you can do this with a jointer. Finish the jointing by smoothing it with a paintbrush. As a decorative thought, make sure to alternate stones with long and short faces to prevent continuous lines that could cut the fireplace in half. If the length of a stone matches one set near it, move it and get a longer corner stone. Keep color in mind for a well-rounded look as well.