I'm assuming the tinfoil-ish cap is in place to keep the e-liquid from drying out. You'll need to take that off entirely. You should be left with a plastic mouthpiece containing filler material. Remove that filler, wash, and dry it (hopefully getting out all the flavor of the juice that is currently saturating it).
I haven't personally been all that successful at cleaning used filler, so I just replace it entirely. If you live near a Wal-mart (or any store with pet supplies), purchase a small box of Fluval (located in the Aquarium section). Strip everything out of your cartridges; wash in soapy water, and dry; then replace the original filler with the clean, new filler. If needed, you can find videos on Youtube that will show you exactly how to do this. In any case, your cartridges will still work (whether you choose to clean the original filler or to replace it entirely).
If anything I'm saying is incorrect, hopefully someone will come along soon to point it out and further clear things up for you.
