For mouth to lung, you first inhale the vapor into your mouth and then inhale into your lungs. This method is best for PG e-liquids, has less of a throat hit and more flavor. For direct lung inhale, you would inhale straight to your lungs. This method is best for cloud chasing and has a more intense hit.
To use e-cigs, you would either use the e-cig until it’s finished and throw away (disposable) or use and then replace the cartridge (replaceable). Closed pods are simple to use by simply popping in a new cartridge when the old one is done and draw.
To use vape pens, all you have to do is refill the tank with e-liquid. To turn on or off, simply press the firing button 5 times and to use, hold the fire button while drawing.
For sub-ohm tanks in vape mods, you can fill them from either the top or bottom. To fill, take the dropper from the e-liquid bottle and fill at an angle. After that, you are all set.
For more info, read- how to use an eCig & e-Liquid Vaporizers