Mix the Mortar
Cut a bag of thinset mortar in half using your trowel (Image 1). Half of an 80 lb. bag is enough for the hearth, legs, and header of one fireplace. If you are setting two fireplaces, mix the entire bag.
Add about a cup of clean water to a clean five-gallon bucket. This will prevent the mortar from sticking to the bottom of the bucket.
Add the thinset slowly into the bucket. Also add water in small amounts at a time – you don’t want to end up with a watery mix. Use a trowel or power mixer to stir the mixture until there are no lumps and it has a consistency of toothpaste (Image 2).
Set the mixture aside for fifteen minutes to allow it to “slake,” then remix.
The thinset mortar is now ready to be used. Once mixed, thinset remains in a workable state for up to eight hours. When it begins to dry out, do not use it, and do not add water – it will only weaken the mortar’s bond.