Before you decide to use home mixed or cartridge fireplace mortar, consider the pros and cons of each.
Home Mixed Mortar
Home mixed mortar is exactly what the name says. It comes in a box or tub, and you must add water to it and stir until it is well-blended and the consistency you need. If you are a novice, this can be challenging. It is very easy to get it too thin, and it won't stay in place until it's dry. On the other hand, if it is too thick, it doesn't spread and fill as easily, and it may crack as it dries. It also creates dust when you are stirring it.
The main advantage of this mortar is its less expensive to use, especially if you are doing a big job.
Cartridge mortar comes in tubes like caulking, and you use the same applicator that you do for caulking. Some comes in tubes that can just be squeezed. It is pre-mixed to the proper thickness and is ready to use. It is very good to use if you are repairing or replacing mortar in small areas. You simply cut the end of the tube, apply and clean.
The disadvantage is primarily cost as it is more expensive than home mixed mortar.