How do I get from SMN train station, Florence to Santa Croce Square?
If you have luggage with you, then a taxi is the best option. It's walking distance and there are no hills, but a bit of a long walk (at least a mile), that you may not want to undertake if you have a lot of luggage. If you have no luggage, then it's a nice stroll through the historical center.
Taxi (recommended) or minibus line C2 (not suitable with large bags, runs only from 7am to 8pm, stops at Via degli Orti Oricellari or Piazza dell'Unità, get down in via Ghibellina).
I agree with the others, Taxi is the best option, especially when hauling luggage.