Easy ?
grindstones, polishing, cutting into smaller pieces... sounds good as long as you have electricity and power tools.
It's obvious they did it anyway, without those tools, but it still seems amazing to me.
Relatively speaking - I did some sculpture in school and we played around with various stones - from Wiki
“Marble also has the advantage that, when first quarried, it is relatively soft and easy to work, refine, and polish.[1] As the finished marble ages, it becomes harder and more durable. Preference to the cheaper and less translucent limestone is based largely on the fineness of marble’s grain, which enables the sculptor to render minute detail in a manner not always possible with limestone; it is also more weather-resistant.”
- and its true, it is easier to carve than basalt for instance (which is a %*^%%), etc.
For rough shaping I assume they used hammers and chisels much like Michelangelo did.
They had saws likewise, I assume used with abrasives.
They had grindstones - i.e., they used stones to grind other stones using abrasives like volcanic sand, etc.
And its not out of the question for them to have used something like a potters wheel - sort of like a vertical lathe.