Hello my fellow wizards!
A few years ago I have created my own technique of doing spiderweb/cobweb effect on miniatures (I'm sure it was done before, but I have developed this technique on my own), using simple, water based glue. After that, a few companies have created spiderweb effects and started selling them. Today, I think it is a good idea to remind everyone, that this effect can be achieved simply, by using cheap products from your local store.
Let's get to the point! All you need to create the spiderwebs effect on your dioramas/miniatures etc. is just clear, waterish glue and your fingers.

Start with pouring some glue onto a palette or any other surface.

Get some of the glue on your fingers (on your pointing finger).

Squeeze your pointing finger and your thumb together.

Go back and forth till you start getting these stringy webs.



Start gently applying your webs onto the surface you want covered in some super realistic spiderwebs. 
The last step of this tutorial is to enjoy your spiderwebs effect :)
Hope you fount this useful!
Cast them spells and paint them minis!
Mamikon