Hi Elgato ... (cute! "The Cat in the Hat", if I remember my Spanish correctly)
I've got 2 Phiniac tanks and have been using them for some time now. LOVE 'em! Like Kay.1959, when I first got them unassembled is the only time both caps are off. I rub juice around the inner upper and lower parts of the tank (inside the tank), and use a Q-tip to also spread juice on the inner o-rings on top & bottom cap. Snap caps back on by first placing them at an angle to the tank, as opposed to putting the cap back on "straight".
I syringe fill my Boge carto like I would any other carto, and blow gently thru the top to get juice out of the center piece. Using a stainless steel Slap Yo Mama (SYM) multi-tool adapter, I put it on top of the filled Boge, and easily push it into the tank through the bottom cap ... and then on up a bit to the very top of the cap. Slides easily without tearing the inner o-rings. So with the SYM small end barely at the top cap, I then fill the tank about 7/8 of the way with juice .. push the carto all the way in from the bottom until the top of the Boge is even with the top of the cap ... remove the SYM and replace with my drip tip.
Some people take 3-4 dry hits on their pv ("vaping" without hitting the button) and THEN sit back and vape themselves silly. I don't bother with the dry hits! Once the whole thing is put together, I jump right in.
If you EVER have questions or problems with the Phiniac, Jennifer & Chris at Phiniac are THE BEST at helping ... and I know because I have pm'd Jennifer numerous times with questions. She has always answered thoroughly and promptly. LOVE those guys.
Hope this helps! Enjoy your Phiniac!
Jackie