how do you guys go about mixing soil?
I check what kind of tree it is, find the soil requirements using Google, and mix with my bare hands.
Bare Hands? Macho Man.
Don't worry about pH, for bonsai and fast draining soils, it is not that important. Mixing soil is different, and if you ask ten different bonsaist, you will get ten different answers. Most importantly, it has to be free draining, for most trees, about 60% inorganic to 40% organic works.
John