The process is quite simple. Your skulls may not be exactly the same as mine, so bear with me if I describe something you don't see.

Materials:
 - Latex Liquid Nails for Projects.  This is sold in two different sizes; a squeeze tube and a caulking gun tube.  The latter is more cost effective.

 - Latex (or other material if you're allergic) gloves

 - craft (popsicle) stick or other small disposable object to aid in spreading the goop.

 - stretchy spiderwebs.

 - Newspapers or other material to protect your work surface.

 - Black and brown acrylic craft paints (optional)

 - Minwax stain (two colors, depending upon the skull, I use are Golden Oak, and Mahogany)    (OPTIONAL)

 - disposable container

 - cheap 1" foam paintbrush (OPTIONAL)

The first thing I do is to remove the "loop" that's on top of the head.    Then I cut out the eye sockets and the "gaps" between the teeth.  I use a Dremel to do this, but I have used a sharp hobby (xacto) knife and it works well if you're dremel-impaired.  These steps are optional, but I prefer the look.

The next step is to clean the skull.  I don't go overboard on the corpse skulls because the Liquid Nails seems to stick to it without a problem.

I applied some black and brown craft paint to my skulls.  This isn't necessary, and as a matter of fact, I recommend that you try your first one without doing it to see of you like the results.  You can always add the paint later.  What I did was to darken up the "shadow" areas, the space in the nose, around and inside the eye sockets.  Then I created a "wash" with the paint.  (simply thin the paint with very little water, brush it on the skull, and then wipe off most of it.)  I did this with brown, and then with black.  It gave the skull an aged look, but again, it wasn't really all that visible after the corpsing, so you may want to skip it.

Now the fun:

pull out some of the spiderwebs.  Using scissors, cut it into smaller, workable pieces.  You want sizes anywhere between 4 and 8 inches.  Cut about 10 or 12 pieces to start with.

Put on your gloves

Open the liquid nails container and squirt some into a disposable
container (I used a petri dish, but you can use a paper plate) and either using the popsicle stick or your gloved hand, stir it around a bit to warm it up and make it more pasty.

Using either the foam brush or your gloved hand (I just use my hands now), spread a thin layer of the LN on an area on top of the skull. There' no need to cover the whole thing yet.  It's best to work with smaller sections until you're in the swing of it.  Let it tack for a few seconds, then take a piece of the spider webs, stick it on to the spot in a clump.  Dip your fingers (or brush) in the LN and spread it on top of the webs.  Now you can begin to spread out and slightly stretch the webs, making the desired shapes and patterns.  Play around with it and get the look you desire.  Remember to start with just a little of the liquid nails.  You want just enough to saturate the webs to be able to form them.  Continue with this process until you've done one side of the skull, then set it down on and unfinished side to dry.  I usually start with the face and work back over the top of the head.

You'll notice that if you squirt out a bunch of the LN ahead of time, it starts to dry up on you a little as you're working.  There's a point when it's perfect and you can use it to mold things like skin flaps and irregularities.  Try applying a dap of it to the skull and "slapping" it with your hand.  You can make some really neat effects with it.  The thing to remember is that there really aren't any rules to it, so have fun and do what looks good to you.

After I corpsify the other half of it and the skull has dried, I apply a stain to it to give it the desired color.  I use a rag to apply the stain, but you can use a brush if you like.  I dip a small part of the rag in the stain, dab it on the skull, and then use another part of the rag to wipe the stain, removing some of it, and smearing it around irregularly.  The dried liquid nails has an organic color to it, so you may not want to stain yours, or you may just want to apply some accent paint.

I have found that straight Liquid Nails produces good results, so you may wish to try that method.  Another method that works pretty well is to tear some white paper towels or toilet paper into irregular pieces and apply that to the LN in place of the webs.  I personally prefer the look of the webs.