Plastic card tubing or rod (as long as it is hollow in the middle)
Something you can use to poke it, like a pin tool or a really tiny plasticard rod.
So first you want to mix up some Green Stuff, the put it down where you want it to go using a flat-ended sculpting tool as pictured behind the model. Then you are going to smooth it down flush with the flesh to make it blend in, leaving a little extra padding where your socket will be.
Then, you get a hollow rod the size you wish your final port to be and you stick it into the green stuff and give it a little wiggle.
Next, you poke the small "nipple" of green stuff left in the middle with your poker (mine was a small plastic rod) and bam, you have a nice fesh socket, which, once you paint all metal with irritated flesh around it, will look pretty cool.