Using the RAMPs board to control additional power supplies

I am working on a 3DP system that needs additional power supplies.... However, I would like to turn off/on these power supplies using the RAMPs board so that I can assign g-codes.

I'm assuming this has been done before, does anyone have any advice where to start? The power I need is too high to pass through the board, which is why it has to be external.

when do you need to turn on/off the power supply?
from slicer, you can add a gcode to run ext1 (if your machine is single extruder) every layer change or something like this.
to physically turn on-off the psu, i suggest you to use a solid state relay to ext1, that close the circuit between the psu and ac.
if you only need to power up the psu, when ramps is only connected to your pc, thats a bit more difficult.
in this case, you need a transistor that is able to drive a relay, and connect this to one of the servo output on ramps, but i have no idea how to use servos and how to control them

Presuming you're using ATX power supplies, you can use an unused pin to control a relay that will short the PS_ON wire to ground for that supply. Use any of the many relay boards or design your own. Solid state or mechanical, doesn't matter. The former is overkill but the latter has mechanical limits although probably won't be stressed by what you'd be using it for.

Using Marlin or Sprinter, use the M42 code to control an individual pin. The format is M42 Px Sy where X is the pin number and y is the value to set the pin at. PWM pins can be 0-255 while digital pins values 0-127 is off and 128-255 is on.