Remove any surround finishing, like marble or tile, that may be affixed to the fireplace. Do this by working the end of a pry bar between the finishing and the wall, prying all around the finishing until it pops off.
Brush the fireplace -- the bricks and the grout joints between them -- with a wire brush. This loosens any weak cement and helps remove any lingering adhesives. Soak a cotton rag in white vinegar and scrub down the entire fireplace, then let it dry for one or two days.
Mix together a batch of white thinset cement and scoop it onto a flat, straight trowel. Wipe down the fireplace, coating it in a layer of thinset that covers the brick and fills the grout joints. Let it dry overnight.
Spread another layer of thinset over the dried layer from the day before, using a notched trowel this time. Press your tiles into the thinset, twisting them slightly as you press them in to ensure a tight grip.