That depends on what you mean by "spindle" and how you want to control it. The spindle configuration should be in the spindle manual. A simple DC motor is one thing, it just needs you relay to switch on the power, but if you want to control the RPM then you have to set up Mach3 for PWM and that is as far as I remember very well described in the manual of Mach3. If you want to control a 3-phase brushless spindle, which uses a VFD then you have to configure the VFD as well, and that is different for different VFD. The Mach3 part is the same as for the DC motor. Then again, if you want to use Modbus you will need to consult the VFD manual. Here, the part which is for configuring Mach3 is not well documented, but there are bits and pieces you could join together to get it right.
You should not look for reference information on Youtube, better read the manual of each concerned product.