Do you have a fill port or not?
Some people put the fill port at the bottom of the tank (toward the battery end), then fill the tank upside down. Then put back in the plug, invert the tank and watch for bubbles to come out of the holes to show that the filler is getting saturated. Check for drips and wipe if necessary before screwing on batt.
I usually put the fill hole towards the top of the tank. I use a .... plug to seal up the battery hole, take out the fill hole plug and fill the tank while it is lying on it's side. Once the tank is full I put the fill hole plug back in and take the .... plug out (in that order, or juice will leak out). Let any excess drip out (sometimes happens, sometimes doesn't) and then give the battery end a wipe before screwing it on the batt..
I've tried it both upside down and right side up, and I don't really get any different results either way.
If you don't have a fill port, you slide a syringe needle between one of the rubber cap plugs and fill that way (how he does it in the video).