Hi!
Flagstones of Trokair works just like a fetchland for this mono coloured versions, to thin out the deck. This is an aggressive version of Soul Sisters with a low mana curve, that's why this works out pretty well. Due to it being legendary, you can fetch for a
Plains when you play one and if you already got one on the battlefield - just don't
forget to tap your first one for mana before you play the second one. You won't lose any mana that way. And yes, you can use
Ghost Quarter on your own
Flagstones of Trokair aswell, for example if you need tripple white for
Spectral Procession on turn 3. Both lands synergyze pretty well together. Also, having a playset of
Ghost Quarter in the maindeck is quite good at the moment because of the emergence of Tron in the current metagame.
cheers