My current project needs one Nema 23 stepper with a big black external driver. Big Black Driver
The CNC shield has Dir, Step and Enable. CNC Shield V3
However, the big black driver has:
DIR +, DIR -
PUL+,Pul -
Enable +, Enable-
What wires do I actually need to go from the CNC shield to the external driver?
Pololu might have some help.
The datasheet would have been the best so a hopefully good guess is:
Connect the 3 minus to GND and the three poisitive to the controller outputs.
Didn't it occur to you to type "CNC shield + tb6600 Arduino" into the Google search bar? 
Did you understand the data sheet? It tells you this:
Connect PUL+, DIR+, EN+ to the power supply of the control system. If the power supply is +5V, it can be connected directly, if the power supply is greater than +5V, an external current limiting resistor R must be added to ensure that the driver's internal optocoupler is powered by 8-15mA power. The pulse input signal is connected via CP. The pulse input signal is connected via CP, the direction signal via DIR and the release signal via EN.
PUL, DIR, EN - connect to the mass of the control system; pulse output. The input signal is connected via PUL+, the direction signal via DIR+ and the release signal via EN+. If a current limiting resistor is required, the connection method of the current limiting resistor R is the same as that of the common anode.
As far as the CNC shield put it back in the box, you can connect the Arduino directly to the black box.
