I'm really stumped with this one.

How do you blend riffs with chords?
As in, the lead guitar with the rhythm guitar?

For example:
I have a riff.
How would I blend it with chords from the rhythm guitar to back up the riff and give it a more "fuller" sound?

1. Look up songs that do that.
2. Listen and check out the tab if you need to.
3. Apply the different techniques to your stuff.
see what chords go with the notes in your riff.

if your riff contains E-G-B-D

you could play E minor or Em7 because you riff is made up of notes that the Em7 chord contains.

this is obviously a very basic way to do this but you should get a firm grasp of it before you move on to something more complex.
I would check out Crazy Train as a good example. It's very "fill-y", and has lots of single note bits mixed in with the chords, especially during the pre chorus and chorus.