There are certain rules I learned that have not let me down to this day when it comes to carto tanks:
CONCERNING THE CARTO - PRIMING:
Drip up to 30 drops of juice into the top of the carto. Don't drop directly down into the carto. Tilt it at an angle and go N E S W, or, in other words, don't drip in the same spot over and over. Twist the carto as you drip to evenly disperse the drops. Wait to watch the juice to soak in. After thirty drops, your carto filler should look like chilled ice. (FIRST MISTAKE: If you think you overfilled it, use it like a straw and GENTLY GENTLY GENTLY blow. If juice comes out of the bottom, you overfilled it, and GENTLY GENTLY GENTLY blow it out until it's gone.) Don't freakin' blow your lungs out (especially if you're ......!)
30 drops should sufficiently prime a carto.
SLIDE THE CARTO BACK UP INTO THE TANK FROM THE BOTTOM
DO NOT LOCK IT UP TOP YET!!!!!!!
Use your intuition when doing this next step; trying to type it up will make you and me both feel stupid...
Fill up the tank. At an angle. Don't get juice back into the carto.
LOCK THE CARTO UP. Put on your drip tip.
Screw the now primed and filled carto tank into your PV.
Take a couple (A COUPLE) of dry hits (aka, draw through your PV like you would if vaping, but don't press the fire button).
You should see bubbles coming out of that little hole in the carto. THIS IS NORMAL, IT IS OK.
Let the carto soak for a couple of minutes. Yes, more than one.
Fire it. It may taste funny for a second or two, but keep drawing. You'll know instantly if you did it wrong or not.
After that, if you're getting good draws, hits, vapor, and flavor, you're in like Flynn. All you have to do after that is keep the tank filled, and there are numerous ingenious devices made for this, and they are all super cheap.
WARNING WARNING WARNING
Do NOT lift off the top cap of the tank to add more juice. If you do so, you will most DEFINTELY flood your carto, as the air pressure from pushing the top cap back on will force air into your cartos, pushing juice where you don't want it to go. ALWAYS slide the carto down into the tank and fill from there, at an angle.
All of this took a while to type, but I learned the technique within five minutes, and it hasn't failed me yet.