Start by painting your spiders with two fairly thick coats of the Craft Twinkles paint.  Be sure to allow to dry before adding a second coat.  Allow spiders to dry overnight before touching.  I only painted the top and sides of my spiders.  The bottom is against the bag and will not be seen anyway.  Be sure to protect your work surface as you can see below.

Stick the spider web to the front of the bag and glue on the painted spider with hot glue.  I cut my spider webs from vinyl on my Cricut Explore.  Give this spiderweb SVG a try if you decide to use your machine. You could paint or draw on a similar design.  If you are lucky enough, a similar spider web might be at your dollar store.