The visible vapor created from an is created by warming a liquid nicotine solution, often referred to as e-liquid.  E-liquid can be added by using pre-filled cartridges, or empty cartridges filled by an e-liquid bottle.

Most e-liquid bottles have dropper caps where a person can simply drip liquid from the bottle directly into the cartridge until it is wet and visibly full.  This is the most recommended method of refilling.  In addition you could use a blunt-tipped syringe to inject liquid into the cartridge, ensuring it to be completely filled.  The injection method can also be used to fill cartomizers.

Cartomizers models can be filled by dripping, and allowing time to let the liquid sit and sink into the filling.  Injecting works well with cartos because you can more quickly fill the unit through to the bottom without waiting for the liquid to sink in.  There is also the carto-cap method of refilling cartomizers, where the rubber seals on the device are used to fill the cartomizer.  Although there are various ways to fill a cartomizer, different people prefer different methods. Our advice is this: Make sure you keep them completely wet at all times, this will increase the life of the unit, and avoid the burned taste.  Dr. Vapor put out an excellent video quite some time ago about various methods of refilling cartomizers.  We've included a video of our preferred method, and additional video tutorial method links below.

Note: For 510 model cartomizers, please remove the rubber mouth-cap and fill from the mouthpiece end of the cartomizer, not the threaded connector end.

 

Additional Methods:
Care and Feeding of Cartomizers: Part 1
Care and Feeding of Cartomizers: Part 2
 
There are methods of dripping e-liquid out of a bottle directly into the atomizer, this is called "dripping."  This can be very harmful to the product, since liquid can be overfilled and leaked into the battery. In most cases this tends to be the preferred method of keeping a PV filled, but due to the damage it may cause, dripping will void any warranties on most products.  You should only attempt dripping once you have a good feel of how the e-cigarette works, and are not satisfied with cartridges or cartomizers.
 

note: most manual batteries are sealed where they connect to the atomizer, protecting it from liquid leaking in.  Always be sure to check that no liquid has leaked from the atomizer before activating the battery.  You will want to avoid liquid getting into the center hole, in the middle post of the battery connector.  This can damage the switch and destroy the battery, or in extreme cases, cause it to explode.