The Ego should have 3-4 parts.
The battery, which will be the largest part, has a button and is threaded
The atomizer, which will be a smaller, hollow threaded metal thingy. Before you use this for the first time put a piece of paper towel over the threaded end of the atomizer and blow into the open end to blow out the liquid it was shipped in. It taste like .... blow until you stop getting juice on the paper towel. If you HAVE juice, drip about 2 drops of the juice onto the atomizer bridge after blowing it out, and before inserting the cartridge. The bridge is the raised mesh arch you will see inside. this helps prime it for use.
The cone, which is hollow, threaded and cone shaped (you may not have one of these, it's mostly a cosmetic feature)
and the cartridge, which is generally plastic, hollow, and filled with some kind of filler ( probably kind of looks like cotton)
the atomizer screws into the battery, and the cartridge just pushes into the atomizer, open end to open end. If you look inside the atomizer you will see a lil rectangle shape in there, try to line that rectangle up with the rectangular open end of your cartridge.
If the filler in your cartridge looks wet, it came prefilled, just pop it on and start vaping, if it is dry, you will need to drop some juice into the filler. different carts can hold different amounts of fluid, just drip it in until it looks saturated, but is not puddling. It sometimes helps to slide a toothpick or needle or something in between the cart filler and the cart casing while filling.
The cone goes on OVER the atomizer and screws on, It is mostly a cosmetic piece to make it look pretty when assembled, but does alter airflow a bit and many people prefer the vaping experience with it on.
You probably don't want to inhale the very first puff or two on a brand new atomizer, even after blowing it out. It will probably still taste like crap
