DIAGNOSIS Overflow happens because you're pouring too fast. The tanks have a small internal air channel that needs to equalize pressure as ink goes in. If you pour fast, air can't escape and it pushes the ink right back out the fill hole.
FIX Slow and steady wins the race. 1) Open the front cover and the ink tank cover. 2) Take the bottle of Canon-brand ink for your model. Remove the seal and the inner cap. 3) Tilt the bottle and gently place the tip into the matching color's fill port. *Don't squeeze.* 4) Just let the ink gravity-feed in. It will stop flowing when the tank is nearly full. 5) Wait a few seconds for any bubbles to rise, then top it off until the ink level is between the MAX and MIN lines. That's it.
IF THAT DIDN'T WORK If ink still backs up, the tiny air vent in the tank cap might be clogged. Remove the ink tank (lift straight up), then remove the black cap. Look for dried ink blocking the small hole in the center of the cap. Clean it with a pin or compressed air.
HEADS UP Always use Canon GI-series ink bottles for the MegaTank. Third-party inks might not have the right viscosity or chemical properties and can cause clogs or damage that isn't covered by warranty. It's not worth the risk for the few bucks you might save.