mortar mix recipe

I want to make some mortar to stick some pieces of flagstone to vertical cinderblock surfaces, and to place them horizontally, too. The horizontal would be capping, and laying on top of other pieces of flagstone.
What is a good recipe for mortar that will stick? Or should I just use the premix stuff at Home Depot?
Steve
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You may want premix thinset mortar, but I am not certain. That is what I used to lay slate tiles onto backerboard for a woodstove hearth, but it was an interior application. I also used a special additive - forgot the name - that makes the mortar stand up in points, and flow less - better consistency.
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SteveB wrote:

Unless you have a *lot* to do and want to save a little money, just use standard, packaged mason's mortar from wherever. Use type S.
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