I know there are different kinds of tanks.. But if it is like mine- when you look at the clear tube: near the bottom of it you should see the O-ring inside the tube. Just below that (2mm) is the place that the tube connects to the (metal) cap - most are very snugg so the tank won't come apart, but if you can ever-so-slightly lift the tube from the cap (I use my finger nail to bairly lift one side - then push it back down) you can prime the carto. Bubbles indicate that Niquid is flowing in. It may not be the best way to initially fill the carto, but it works really nicely when the carto is not priming itself well.
Hope this helps
I would post a photo if someone can tell me an easy way to do it from my iPhone?
Post is too vague, way too many brands and kinds of tanks to post something like this without a pic. How has every method known to man to fill a tank not been mentioned by now, they have been out forever!