Allright, done!
I’m putting it in spoilers, because there are a whole bunch of pictures.
Masking allows you to force one item not to go outside the boundries of another item.
Here’s an example:
In example A there is no masking, but in example B the glove has been masked to the arm. Note also how the outlines of the arm shows through the masked glove. This is another effect of masking.
The objects don’t lock together, though. If you were to move the glove in example A away from the arm it would disappear from sight.
There are a few different methods to clothing turned bodies. You can mask the clothes onto the Body items, or you can simply colour the items so that they look like skin tight clothing. Here’s one method that I personally like to use, which is a tad more complicated.
Let’s say we want to put a jacket on this half profile torso. Start by adding two jackets of the same kind.
Now widen one of the jackets by about 20% and narrow the other one by the same amount. (The amount varies depending on the chosen article of clothing, but 20% is what I used for this example.)
Place the wider jacket over the left side of the torso until the left side of the jacket fits. You may have to rotate it slightly. You may also want to make the jacket transparent to make it easier to see what you’re doing.
Choose a square insignia from either the Insignia-Standard or the Insignia-Tech section. Enlarge it until it covers the left side of the jacket.
Select the jacket, click “Mask” and then click the square.
Repeat the process with the other jacket for the right side of the torso.
Layer the insignias behind the torso and the jackets, and there you go!
Let’s say we want to give the guy a shirt, too. Choose one that you like.
Place it on the torso, layered under the jackets.
Now let’s get rid of those sleeves and bits that stick out to the sides. Get a third square insignia, and place it over the parts of the shirt you want to be visible.
Mask the shirt onto the square, and layer the square to the back.
Here I repeated what I did with the jackets with two hats. Make one wider and one more narrow, mask them to two different squares.
An item can only have one single item masked to it, but a masked item can have another item masked to itself. The shirt was masked to a square, but there is no problem masking an insignia to the shirt.
Let’s say you do want to mask several items to one single object. Well, there is a way to kind of cheat your way to that. In the below example I’ve added an arm to the guy, and masked an insignia of a bird to it, as a tattoo. To add another tattoo to the same arm I added another arm (a transparent blue one), and placed it on top of the first one.
Then I masked a cross insignia to the second arm…
… and placed the blue arm behind the first one.
I also added a background here, to cover the insignias used for masking. You can also make the insignias completely transparent, but I would advice you to wait until the design is finished before you do that, since it’s always good to know where they are, if you need to change something.
The colours used for the insignias in this tutorial were mainly for illustratory purposes, but using different colours can really help you make out which is which if you’re using a lot of them at the same time.
Hope this helped 
Here are two tutorials made by Jeff Hebert for masking items into hands and masking patterns, neither of which I talked about in the above tutorial, but are good things to know nonetheless.
