Here's how I made mine.
 

 Good pliers, Hot Glue Gun with many glue sticks, and Snips or Scissors.

Be careful when applying the hot glue on the spider sac.  It will be HOT and you don't want to burn your fingers while gluing together two edges of fabric!

Sprinkle dirt and debris from your shop floor on the spider sac for a more realistic look.  Add moss or straw to the sides holding it on with a little hot glue.

Be sure to wrap enough shop cloth and cheese cloth around the chicken wire to hide the look of the wire form.

The Chicken Wire doesn't have to be a certain size.  I just had this piece already cut out.

This piece was about 1 ft. x 1 ft.

Be sure to cut your piece of chicken wire square, but it can be whatever size you want your spider sac to become.

Take the square chicken wire and fold it over to connect opposite side of the square.

Use pliers to twist the chicken wire onto itself.

Once you secure opposite sides, begin to secure the chicken wire.  You want to make a cone shape from the once square wire.

Your chicken wire will now look somewhat cone shaped.  This is the look we want.

You will want to enclose the bottom of the cone shaped chicken wire.  I pinched the sides, folded over the excess, and lipped it back over the side to make a rounded bottom.

Do what you must, but I believe the spider sac should be round when you're finished.

With the pliers, I made a loop from the chicken wire.

The loop is needed to attach a hanging string later in the build.

Just make sure your loop is sturdy enough to handle the weight of the spider sac.

I used my hands to rip up the cheese cloth.  Then dragged everything across the floor to dirty it up a bit.

Use the hot glue gun to adhere the pieces of shop towel to the chicken wire sac.

Notice how you can see through the cloth, which makes the wire very visible?  That's something we want to fix.

Use at least two coats of shop towel around the chicken wire frame so that the chicken wire is covered well.

The cheese cloth will go on next.  Use ample amounts of hot glue during both the towel and the cheese cloth wrappings to keep it all in place.

Be sure to use the hot glue as spider webbing too!  When the glue drips, quickly wrap it all around the spider sac.

NOTE:  THE HOT GLUE GUN WILL BE HOT!  BE CAREFUL WHILE APPLYING THE SHOP TOWELS AND CHEESE CLOTH.  ALLOW TIME FOR THE HOT GLUE TO CURE BEFORE ATTEMPTING TO TOUCH IT WITH YOUR HANDS!  CAUTION!  DO NOT HANDLE HOT GLUE WITH YOUR BARE HANDS!

It's hard to see, but I used the hot glue gun to make many drips and "veins" on the spider sac.

It's perfectly normal to go a little crazy on this part.  Drip hot glue all around the cheese cloth.  In and over the edges of the towels and cheese cloth.

Just don't burn your hands!  Please be careful using the hot glue gun.  I burned myself several times, and I was going very slow and being very careful.  I suggest gluing a small section of the spider sac at a time, let it cool, and begin again.

NOTE:  THE HOT GLUE GUN WILL BE HOT!  BE CAREFUL WHILE APPLYING THE SHOP TOWELS AND CHEESE CLOTH.  ALLOW TIME FOR THE HOT GLUE TO CURE BEFORE ATTEMPTING TO TOUCH IT WITH YOUR HANDS!  CAUTION!  DO NOT HANDLE HOT GLUE WITH YOUR BARE HANDS!

After hot gluing the entire spider sac, I thought it looked a little too neat and perfect.

I took out my tin snips (or scissors) and cut up some of the cheese cloth so it could hang off the spider sac.

I also added some more cheese cloth that I tore up nicely with my hands to the bottom and trimmed it as well.

Add some character by throwing a little dirt and debris on your nice white spider sac.  It will make it look a whole lot more realistic.

I just swept up some dirt off my shop floor and threw it on the spider  sac randomly.  Just don't cover too much of the white up, or it may not look like a spider sac.

I also added some hay strings and glued them down with my hot glue gun.

Moss would make a nice touch, as well, and of course add some of those tiny spider rings that you probably already have from last year.