Thyestean (who just swooped in front of me here

) has provided a link to a very relevant thread. That is, his
first link; the second is not working.
In addition, take a look at
this recent thread and
my post within it.
A two-piece cartomizer e-cig
is the simplest (not that other types are complex or difficult). But Green Smoke (as well as other brand names for the E9/V9 model) is much more
expensive than the KR808D-1 model (which also has myriad brand names).
If voluminous vapor is of first, second, and third importance to you, Green Smoke (or its E9/V9 cousins) might be worth the extra cost. Otherwise...?