technique - How to do the ghost mannequin effect?

I'm looking into taking some product photography and I would like to get the ghost mannequin effect, but I can't find any proper documentation.

Something a bit like this:

Can you please give a stepped process (ie 1,2,3) (don't need too many details) .

Thanks for any advice.

Answer

Thanks for all the feedback. Mixing and matching what other answers that were given, I got this.

Mixing it all together a bit, here is a very very quick snapshop of what I'm going to do. Please note that I did this in 5 min, didn't take out all the gear...and only did the neck part for demonstration.

STEP 1: Take a simple picture picture of your item on a mannequin:

STEP 2: Take a picture of your t-shirt inside out on the same mannequin

STEP 3: Mask out the neck part of the picture you took in step 1:

STEP 4: Mask out the picture you took in step 2 and add some shadow:

STEP 5: Put your step 4 underneath your step 3, then simply stamp or heal tool where you need to get the result:

This is pretty time intensive editing. But it can give awesome results.