Lots of folks consider cartos as consumables and just toss them - others, more frugal, clean them for reuse.
First you want to remove the existing fluid - this is fairly easy using a taryn spin. Just remove the drip-tip/stopper from the mouth end, fit a rubber condom (the kind that come with the new carto) over the mouth end but only on as far as necessary to hold it there, tye a string around the battery threads and spin it like a sling to force the fluid out of the filler into the condom. You could save or just toss that juice. Now you're ready to clean the filler and there are several approaches.
There are several vendors that sell plunger kits to flush fluid thru the carto to clean it: many users flush first with water then flush with PGA (to drive out the water, clean the coils, and allow the filling to dry quicker). I personally have found this method works best for me: it's quick and seems to do a good job.
Some use mouthwash to remove lingering flavors.
Some soak in hot water, others soak in alcohol - I've heard of both PGA and IPA used (but there has been some disagreement over using IPA). Some actually boil the cartos in water (I understand some say boiling can ruin a carto by quickly inducing rust and can leave deposits inside the tube).
Some folk swear by an ultrasonic cleaner filled with mouthwash to clean cartos (others swear at it, saying the vibration weakens/breaks the coils prematurely).