Hi,
page 16 of the 57CNC manual shows the pinout of the 8 pin motor sockets.
You will need the Enable pin1, Dir pin3, Step pin5 and if you want the servo to be able to signal an error back to Mach then Error pin7.
Pins 1,3,5 are all DO5 type pins, 5V output, 100Ohm impedance, 8mA max current per page 19 of the manual. Should be able to drive the input opto couplers
directly. The Error input is DI5P type, 5V tolerant, pulled high by 10kOhm, page 19, and can be connected to the Alm+ output of the servo driver.
The connection is simple.
Do you really want those servos? Get a torque vs speed diagram before you buy them. If they can't supply one then stay away from that brand.
Craig