you can directly fix the terracotta in to sand and cement but you would need to soak the tiles over night and allow them to drain 20 minutes before fixing them , you cant use a too strong a mix on terracotta as the more cement added the more shrinkage you get in the mortar and this can crack the tiles , I would go 5;1 sharp sand to cement and enhance it with sbr you will need to slurry the backs of the tiles in order to bond them to the screed, the advice Above is probably the best if you have no experience in fixing in mortar beds screed first let this dry then tile with powdered adhesive