If you want a spiderweb that is more realistic than those bags of fuzz that you get at the store, you can pay about $50 for a special webcasting gun or you can use your regular old hot glue gun! Just keep in mind that it may be difficult to get off of some items so you should only use it on your Halloween props.
Start by making a bunch of Paper Roses using black or red cardstock. Use hot glue to attach the roses to pieces of 18 gauge wire.
Grab a dollar store candle holder and cut down a piece of styrofoam to fit inside. Place a strip of black cardstock around the styrofoam to hide it. Start poking the wire rose stems in the styrofoam.
Keep going until your bouquet is full.
For the spiderweb, get your glue gun hot and squeeze the trigger so that there is just a little bit of glue coming out of the tip. Tap the tip on the edge of one of your roses and gently pull the gun across the top so that you have a fine string of glue laying over the top. Continue doing this going in all different directions until you have a satisfactory amount of coverage.
Pop on a couple of spiders and you are done! Hopefully I will have time to make some more realistic spiders but for now the cheap plastic ones will have to do.